One day together, for pastime, we read
Of Lancelot, and how Love held him in thrall.
We were alone, and without any dread.
Sometimes our eyes, at the word’s secret call,
Met, and our cheeks a changing color wore.
But it was one page only that did all.
When we read how that smile, so thirsted for,
Was kissed by such a lover, he that may
Never from me be separated more
All trembling kissed my mouth. The book I say
Was a Galahalt to us, and he beside
that wrote that book. We read no more that day.

[Noi leggiavamo un giorno per diletto
di Lancialotto come amor lo strinse;
soli eravamo e sanza alcun sospetto.
Per più fïate li occhi ci sospinse
quella lettura, e scolorocci il viso;
ma solo un punto fu quel che ci vinse.
Quando leggemmo il disïato riso
esser basciato da cotanto amante,
questi, che mai da me non fia diviso,
la bocca mi basciò tutto tremante.
Galeotto fu ‘l libro e chi lo scrisse:
quel giorno più non vi leggemmo avante.]

Dante Alighieri (1265–1321) Italian poet

The Divine Comedy [Divina Commedia], Book 1 “Inferno,” Canto 5, l. 127ff (5.127-138) [Francesca] (1309) [tr. Binyon (1943)]
    (Source)

In the Old French romance of Lancelot du Lac they were reading, Sir Gallehault (spelled variously) serves as go-between for Lancelot and Guinevere (a couple not able to express their love because of her marriage to King Arthur), and ultimately persuades the Queen to give Lancelot a first, dooming kiss. Similarly, Paolo was the intermediary to arrange the marriage of his brother, Gianciotto, and Francesca. After the marriage, reading together that racy tale of Lancelot seduced Paolo and Francesca into pursuing their carnal affair.

The Italian form of Gallehault -- "Galeotto" or "Galleot" -- became Middle Ages Italian slang for a panderer or pimp, and Francesca draws on this meaning in her chat with the Pilgrim, blaming the book and its writer for her damning sins with Paolo. See also, earlier, here.

(Source (Italian)). Other translations:

Together we, for pleasure, one day read
How strictly Lancelot was bound by love;
We, then alone, without suspicion were:
T'admire each other, often from the book
Our eyes were ta'en, and oft our colour chang'd;
That was the point of time which conqurer'd us,
When, reading that her captivating smile
Was by the Lover the adored kiss'd;
This, my Companion, always with me seen,
Fearful, and trembling, also kiss'd my mouth.
The Writer, Galeotto, nam;d the Book.
But from that day we never read in't more.
[tr. Rogers (1782), l. 113ff]

One day (a day I ever must deplore!)
The gentle youth, to spend a vacant hour,
To me the soft seducing story read,
Of Launcelot and fair Geneura's love,
While fascinating all the quiet grove
Fallacious Peace her snares around us spread.
Too much I found th' insidious volume charm,
And Paulo's mantling blushes rising warm;
Still as he read the guilty secret told:
Soon from the line his eyes began to stray;
Soon did my yielding looks my heart betray,
Nor needed words our wishes to unfold.
Eager to realize the story'd bless,
Trembling he snatch'd the half resented kiss,
To ill soon lesson'd by the pandar-page!
Vile pandar-page! it smooth'd the paths of shame.
[tr. Boyd (1802), st. 24-26]

One day
For our delight we read of Lancelot,
How him love thrall’d. Alone we were, and no
Suspicion near us. Ofttimes by that reading
Our eyes were drawn together, and the hue
Fled from our alter’d cheek. But at one point
Alone we fell. When of that smile we read,
The wished smile, rapturously kiss’d
By one so deep in love, then he, who ne’er
From me shall separate, at once my lips
All trembling kiss’d. The book and writer both
Were love’s purveyors. In its leaves that day
We read no more.
[tr. Cary (1814)]

'Twas on a day when we for pastime read
Of Lancelot, whom love ensnared to ruin:
We were alone, nor knew suspicious dread.
That lesson oft, the conscious look renewing,
Held us suspense, and turned our cheeks to white;
But one sole moment wrought for our undoing:
When of the kiss we read, from smile so bright.
So coveted, that such true-lover bore.
He, from my side who ne'er may disunite,
Kissed me upon the mouth, trembling all o'er.
The broker of our Vows, it was the lay,
And he who wrote -- that day we read no more.
[tr. Dayman (1843)]

One day, for pastime, wwe read of Lancelot, how love restrained him; we were alone, and without all suspicion.
Several times that reading urged our eyes to meet, and changed the color of our faces; but one moment alone it was that overcame us.
When we read how the fond smile was kissed by such a lover, he, who shall never be divided from me,
kissed my mouth all trembling: the book, and he who wrote it, was a Galeotto; that day we read in it no farther.
[tr. Carlyle (1849)]

We were reading one day, for our delight,
In Lancilotto, bound in love so strict.
We were alone, and neither could suspect
Suspended were our eyes, and more than once,
In reading, and the visage colorless;
One point it was lone that conquered us.
When we read first of that -- the longed-for smile
At being kissed by one who loved so well;
Galeotti was the book -- he wrote it:
That Day we read not there any farther.
[tr. Bannerman (1850)]

One day we read, to pass a pleasant time,
How Lancelot was bound in chains of love;
Alone we were and no suspicion knew.
often we sigh'd; and as we read our eyes
Each other sought, the color fled our cheeks;
But we were vanquish'd by one point alone.
When we had read how the smile long desir'd
Was kiss'd by him who lov'd with such deep love,
This one, from me no more to be apart,
Trembling all over, kiss'd me on the mouth.
Galeotto was the writer and the book;
In it we read no further on that day.
[tr. Johnston (1867)]

One day we reading were for our delight
⁠Of Launcelot, how Love did him enthrall.
⁠Alone we were and without any fear.
Full many a time our eyes together drew
⁠That reading, and drove the color from our faces;
⁠But one point only was it that o'ercame us.
Whenas we read of the much longed-for smile
⁠Being by such a noble lover kissed,
⁠This one, who ne'er from me shall be divided,
Kissed me upon the mouth all palpitating.
⁠Galeotto was the book and he who wrote it.
⁠That day no farther did we read therein.
[tr. Longfellow (1867)]

We were reading one day, for delight, of Lancelot, how Love constrained him; alone were we, and without any suspicion. Many times did that reading impel our eyes, and change the hue of our visages; but one point only was it that overcame us. When we read that the wished-for smile was kissed by such a lover, this one who never from me shall be parted kissed me on the mouth all trembling. A Gallehault was the book, and he who wrote it. That day we read no further in it.
[tr. Butler (1885)]

We read one day for pleasure, in the song
Of Launcelot, how Love him captive made;
We were alone without one thought of wrong.
Many and many a time our eyes delayed
The reading, and our faces paled apart;
One point alone it was that us betrayed.
In reading of that worshipt smile o' the heart,
Kissed by such lover on her lips' red core.
This one, who never more from me must part,
Kissed me upon the mouth, trembling all o'er:
For us our Galeotto was that book;
That day we did not read it any more.
[tr. Minchin (1885)]

We were reading one day, for delight, of Lancelot, how love constrained him. We were alone and without any suspicion. Many times that reading made us lift our eyes, and took the color from our faces, but only one point was that which overcame us. When we read of the longed-for smile being kissed by such a lover, this one, who never from me shall be divided, kissed my mouth all trembling. Galahaut was the book, and he who wrote it. That day we read in it no farther.
[tr. Norton (1892)]

We read one day, to while the hour, of Lancelot, how love enthralled him: we were alone, with never a thought of harm. And oft and oft that reading brought our eyes together and drave the colour to our cheeks ; but one point, only one, it was that overcame us. When that we came to read of how the smiling lips he loved were kissed by lover such as he, he that no more shall e'er be parted from me, kissed my mouth trembling through. Our Galahad was the book and he that penned it: that day we read in it no more.
[tr. Sullivan (1893)]

One day, by way of pastime, we were reading
Of Lancelot, how love in fetters held him:
We were alone, and without thought of danger.
Full often did that reading bring together
Our glances, and made colourless our visage;
But just one point was that which overcame us:
When as we read how that the smile much longed for
Was kissed by one so passionately loving,
He who from me shall never be divided
Kissed me upon the mouth, all, all a-quiver: --
A Galehalt was the book and he who wrote it: --
Upon that day we read therein no further.
[tr. Griffith (1908)]

We read one day for pastime of Lancelot, how love constrained him. We were alone and had no misgiving. Many times that reading drew our eyes together and changed the color in our faces, but one point alone it was that mastered us; when we read that the longed-fro smile was kissed by so great a lover, he who never shall be parted from me, all trembling, kissed my mouth. A Galeotto was the book and he that wrote it; that day we read in it no farther.
[tr. Sinclair (1939)]

One day we read for pastime how in thrall
Lord Lancelot lay to love, who loved the Queen;
We were alone -- we thought no harm at all.
As we read on, our eyes met now and then,
And to our cheeks the changing color started,
But just one moment overcame us -- when
We read of the smile, desired of lips long-thwarted,
Such smile, by such a lover kissed away,
He that may never more from me be parted
Trembling all over, kissed my mouth. I say
The book was Galleot, Galleot the complying
Ribald who wrote; we read no more that day.
[tr. Sayers (1949)]

One day for dalliance we read the rhyme
of Lancelot, how love had mastered him.
We were alone with innocence and dim time.
Pause after pause that high old story drew
our eyes together while we blushed and paled;
but it was one soft passage overthrew
our caution and our hearts. For when we read
how her fond smile was kissed by such a lover,
he who is one with me alive and dead
breathed on my lips the tremor of his kiss.
That book, and he who wrote it, was a pander.
That day we read no further.
[tr. Ciardi (1954), l. 124ff]

One day, for pastime, we reqd of Lancelot, how love constrained him; we were alone, suspecting nothing. Several times that reading urged our eyes to meet and too the color from our faces, but one moment alone it was that overcame us. When we read how the longed-for smile was kissed by so great a lover, this one, who never shll be parted from me, kissed my mouth all trembling. A Gallehault was the book and he who wrote it; that day we read no farther in it.
[tr. Singleton (1970)]

One day we read, to pass the time away,
of Lancelot, how he had fallen in love;
we were alone, innocent of suspicion.
Time and again our eyes were brought together
by the book we read; our faces flushed and paled.
To the moment of one line alone we yielded:
it was when we read about those longed-for lips
now being kissed by such a famous lover,
that this one (who shall never leave my side)
then kissed my mouth, and trembled as he did.
The book and its author was our galehot!
That day we read no further.
[tr. Musa (1971)]

One day, to pass the time away, we read
of Lancelot -- how love had overcome him.
We were alone, and we suspected nothing.
And time and time again that reading led
our eyes to meet, and made our faces pale,
and yet one point alone defeated us.
When we had read how the desired smile
was kissed by one who was so true a lover,
this one, who never shall be parted from me,
while all his body trembled, kissed my mouth.
A Gallehault indeed, that book and he
who wrote it, too; that day we read no more.
[tr. Mandelbaum (1980)]

One day, when we were reading, for distraction,
How Lancelot was overcome by love --
We were alone, without any suspicion;
Several times, what we were reading forced
Our eyes to meet, and then we changed color:
But one page only was more than we could bear.
When we read how that smile, so much desired,
Was kissed by such a lover, in the book,
He, who will never be divided from me,
Kissed my mouth, he was trembling as he did so;
The book, the writer played the part of Galahalt:
That day we got no further with our reading.
[tr. Sisson (1981)]

One day, for pleasure,
We read of Lancelot, by love constrained:
Alone, suspecting nothing, at our leisure.
Sometimes at what we read our glances joined,
Looking from the book each to the other's eyes,
And then the color in our faces drained.
But one particular moment alone it was
Defeated us: the longed-for smile, it said,
Was kissed by that most noble lover: at this,
This one, who now will never leave my side,
Kissed my mouth, trembling. A Galeotto, that book!
And so was he who wrote it; that day we read
No further.
[tr. Pinsky (1994), l. 112ff]

We were reading one day, for pleasure, of Lancelot, how Love beset him; we were alone and without any suspicion.
Many times that reading drove our eyes together and turned our faces pale; but one point alone was the one that overpowered us.
When we read that the yearned-for smile was kissed by so great a lover, he, who will never be separated from me,
kissed my mouth all trembling. Galeotto was the book and he who wrote it: that day we read there no further.
[tr. Durling (1996)]

We read, one day, to our delight, of Lancelot and how love constrained him: we were alone and without suspicion. Often those words urged our eyes to meet, and coloured our cheeks, but it was a single moment that undid us. When we read how that lover kissed the beloved smile, he who will never be separated from me, kissed my mouth all trembling. That book was a Galeotto, a pandar, and he who wrote it: that day we read no more.
[tr. Kline (2002)]

One day, to pass the time, we read of
Lancelot, who loved illicitly.
Just the two of us; we had not thought of what, as yet, was not.
From time to time that reading urged our eyes to meet.
and made our faces flush and pale,
but one point in the story changed our lives;
for when we read of how the longed-for smile
was kissed by such a noble knight,
the one who for eternity is by my side all trembling
kissed my trembling mouth.
The man who wrote this was a Galeotto; so was the book.
That day the rest of it remained unscanned.
[tr. Carson (2002)]

One day we read together for pure joy
how Lancelot was taken in Love's palm.
We were alone. We knew no suspicion.
Time after time, the words we read would lift
our eyes and drawn all color from our faces.
A single point, however, vanquished us.
For when at last we read the longed-for smile
of Guinevere -- at last her lover kissed --
he, who from me will never now depart,
touched his kiss, trembling to my open mouth.
This book was Galehault -- pander-penned, the pimp!
That day we read no further down those lines.
[tr. Kirkpatrick (2006)]

One day, to pass the time in pleasure,
we read of Lancelot, how love enthralled him.
We were alone, without the least misgiving.
More than once that reading made our eyes meet
and drained the color from our faces.
Still, it was a single instant overcame us:
When we read how the longed-for smile
was kissed by so renowned a lover, this man,
who never shall be parted from me,
all trembling, kissed me on my mouth.
A Galeotto was the book and he that wrote it.
That day we read in it no further.
[tr. Hollander/Hollander (2007)]

One day we read the story of Lancelot
And how his love attacked and held him tight.
We were alone and unaware of our thoughts.
More than once the story forced our eyes
To meet, and as we looked our faces turned pale,
But just one single moment hung and decided
Us. We read how a smile we longed for stayed
On her lips until the greatest of lovers kissed them,
And then this man, who cannot be taken away
From me, kissed my mouth, his body trembling.
A famous go-between had written that tale.
That day, our time for reading suddenly ended.
[tr. Raffel (2010)]

One day, to amuse ourselves, we were reading
The tales of love-struck Lancelot; we were all alone,
And naively unaware of what could happen.
More than once, while reading, we looked up
And saw the other looking back. We'd blush, then pale,
Then look down again. Until a moment did us in.
We were reading about the longed-for kiss
The great lover gives his Guinevere, when that one
From whom I'll now never be parted,
Trembling, kissed my lips.
That author and his book played the part
Of Gallehault. We read no more that day.
[tr. Bang (2012)]

Reading together one day for delight
Or Lancelot, caught up in Love's sweet snare,
We were alone, with no thought of what might
Occur to us, although we stopped to stare
Sometimes at what we read, and even paled.
But then the moment came we turned a page
And all our powers of resistance failed:
When we read of that great knight in a rage
To kiss the smile he so desired. Paolo,
This one so quiet now, made my mouth still --
Which, loosened by those words, had trembled so --
With his mouth. And right then we lost the will --
For Love can will will's loss, as well you know --
To read on. But let that man take a bow
Who wrote the book we called our Galahad,
The reason nothing can divide us now.
[tr. James (2013), l. 149ff]