Reading good books is like having a conversation with the most distinguished men of past ages — indeed, a rehearsed conversation in which these authors reveal to us only the best of their thoughts.

[Que la lecture de tous les bons livres est comme une conversation avec les plus honnêtes gens des siècles passés, qui en ont été les auteurs, et même une conversation étudiée en laquelle ils ne nous découvrent que les meilleures de leurs pensées.]

René Descartes (1596-1650) French philosopher, mathematician

Discourse on Method [Discours de la méthode], Part 1 (1637) [tr. Cottingham, Stoothoff (1985)]
    (Source)

(Source (French)). Alternate translations:

The reading of good books, is like the conversation with the honestest persons of the past age, who were the Authors of them, and even a studied conversation, wherein they discover to us the best only of their thoughts.
[tr. Newcombe ed. (1649)]

The perusal of all excellent books is, as it were, to interview with the noblest men of past ages, who have written them, and even a studied interview, in which are discovered to us only their choicest thoughts.
[tr. Veitch (1901)]

I was aware that the reading of all good books is indeed like a conversation with the noblest men of past centuries who were the authors of them, nay a carefully studied conversation, in which they reveal to us none but the best of their thoughts.
[tr. Haldane, Ross (1911)]

The reading of all good books is like a conversation with the finest men of past ages, who are their authors, and even a studied conversation in which they unfold to us only the best of their thoughts.
[tr. Kennington (1964-76)]

The reading of good books is like a conversation with the best men of past centuries -- in fact like a prepared conversation, in which they reveal only the best of their thought.
[tr. Ascombe, Geach (1971)]

The reading of all good books is like a conversation with the finest men of past centuries.
[Common translation, unsourced]