The man who mingles the useful with the sweet carries the day by charming his reader and at the same time instructing him. That’s the book to enrich the publisher, to be posted over seas, and to prolong its author’s fame.

[Omne tulit punctum, qui miscuit utile dulci,
lectorem delectando pariterque monendo.
Hic meret aera liber Sosiis, hic et mare transit
et longum noto scriptori prorogat aevum.]

Horace (65–8 BC) Roman poet, satirist, soldier, politician [Quintus Horatius Flaccus]

Epistles [Epistularum, Letters], Book 2, ep. 3 “Art of Poetry [Ars Poetica; To the Pisos],” l. 343ff (2.3.343-346) (19 BC) [tr. Blakeney; ed. Kramer, Jr. (1936)]
    (Source)

Horace advises on how to write a best-seller, by blending both entertainment and (moral) substance. The Sosii were famed booksellers in Rome.

(Source (Latin)). Other translations:

He beares the bell in all respects who good with sweete doth minge:
Who can in delectable style good counsaile with him bring.
His bookes the stationers will bye, beyonte Sea it will goe,
And will conserve the authors name a thowsand yeare, and mo.
[tr. Drant (1567)]

But he hath every suffrage can apply
Sweet mix'd with soure, to his reader, so
As doctrine and delight together goe.
This book will get thee Socij money; this
Will passe the Seas; and long as Nature is
With honour make the far-known Author live.
[tr. Jonson (1640), l. 490ff]

But he that joyns instructions with delight,
Profit with pleasure, carries all the Votes;
These are the Volumes that enrich the Shops,
These pass with admiration through the World,
And bring their Author an Eternal fame.
[tr. Roscommon (1680)]

Profit and pleasure, then, to mix with art,
To inform the judgment, nor to bend the heart,
Shall gain all votes; to booksellers shall raise
No trivial fortune, and across the seas
To distant nations spread the writer's fame,
And with immortal honours crown his name.
[tr. Francis (1747)]

But he who precept with amusement blends,
And charms the fancy while the heart he mends,
Wins every suffrage. Rarely shall he miss
To enrich the Sosii with a piece like this:
Seas shall it traverse, and the writer's page
Hand down his glories to a distant age.
[tr. Howes (1845)]

He who joins the instructive with the agreeable, carries off every vote, by delighting and at the same time admonishing the reader. This book gains money for the Sosii; this crosses the sea, and continues to its renowned author a lasting duration.
[tr. Smart/Buckley (1853)]

But he who, mixing grave and gay, can teach
And yet give pleasure, gains a vote from each:
His works enrich the vendor, cross the sea,
And hand the author down to late posterity.
[tr. Conington (1874)]

He wins all suffrages who, while he charms.
Instructs the soul, the heart to virtue warms,
And so what ministers to use unites
With what is beautiful in all he writes.
These are the works on which the Sosii thrive,
That cross the seas, to times remote survive.
[tr. Martin (1881)]

He meets with acceptance everywhere who blends the practical with the pleasant, by equally delighting and instructing the reader. Such a book enriches the Sosii, travels across the sea, and confers immortality on its famous author.
[tr. Elgood (1893)]

He has won every vote who has blended profit and pleasure, at once delighting and instructing the reader. That is the book to make money for the Sosii; this the one to cross the sea and extend to a distant day its author's fame.
[tr. Fairclough (Loeb) (1926)]

He wins every vote who combines the sweet and the useful,
Charming the reader and warning him equally well.
This book will bring in money for Sosius and Son,
Booksellers, travel across the sea, and extend
Its author's fame a long distance into the future.
[tr. Palmer Bovie (1959)]

The poet winning every vote blends the useful with the sweet,
giving pleasure to his reader while he offers him advice.
His book will make the Sosii money and travel overseas,
and far into the years ahead extend its author's name.
[tr. Fuchs (1977)]

Tame sense with a dash of sugar,
Storke your reader's cheeks while you box his ears.
Then everyone reads you, your royalties mount
Like gushing oil, foreigners run for your latest title
And read you long after you've turned to dust.
So: make your own memorial!
[tr. Raffel (1983 ed.)]

He who provides to all both profit and pleasure
Wins everybody's vote; his book will bring
Money for bookstore owners and fame across
The seas and down the years to the author himself.
[tr. Ferry (2001)]

Everyone votes for the man who mixes wholesome and sweet,
giving his reader an equal blend of help and delight.
That book earns the Sosii money; it crosses the ocean,
winning fame for the author and ensuring long survival.
[tr. Rudd (2005 ed.)]

Who can blend usefulness and sweetness wins every
Vote, at once delighting and teaching the reader.
That’s the book that earns the Sosii money, crosses
The seas, and wins its author fame throughout the ages.
[tr. Kline (2015)]

He wins every hand who mingles profit with pleasure, by delighting and instructing the reader at the same time.
[ed. Bartlett's]