We find a book eloquent not only when it forms our emotions, but also when it fortifies our opinions.
 
[Nous trouvons éloquent dans les livres, non-seulement tout ce qui augmente nos passions, mais aussi tout ce qui augmente nos opinions.]

Joseph Joubert (1754-1824) French moralist, philosopher, essayist, poet

Pensées [Thoughts], ch. 23 “Des Qualités de l’Écrivain [Of the Qualities of Writers],” ¶ 157 (1850 ed.) [tr. Collins (1928), ch. 22]
    (Source)

(Source (French)). Alternate translation:

In books we take for eloquence not only all that strengthens our passions, but also whatever strengthens our opinions. [tr. Lyttelton (1899), ch. 22, ¶ 74]