Diogenes Laertius 2.71 (Life of Aristippus, tr. R.D. Hicks):
When some one gave himself airs for his wide learning, this is what he said: "As those who eat most and take the most exercise are not better in health than those who restrict themselves to what they require, so too it is not wide reading but useful reading that tends to excellence."
σεμνυνομένου τινὸς ἐπὶ πολυμαθείᾳ ἔφη, "ὥσπερ οὐχ οἱ τὰ πλεῖστα ἐσθίοντες καὶ γυμναζόμενοι ὑγιαίνουσι μᾶλλον τῶν τὰ δέοντα προσφερομένων, οὕτως οὐδὲ οἱ πολλὰ ἀλλ' οἱ χρήσιμα ἀναγινώσκοντές εἰσι σπουδαῖοι."
Seneca, Letters to Lucilius
2.2 (tr. Richard M. Gummere):
Be careful, however, lest this reading of many authors and books of every sort may tend to make you discursive and unsteady. You must linger among a limited number of master-thinkers, and digest their works, if you would derive ideas which shall win firm hold in your mind.
Illud autem vide, ne ista lectio auctorum multorum et omnis generis voluminum habeat aliquid vagum et instabile. Certis ingeniis immorari et innutriri oportet, si velis aliquid trahere quod in animo fideliter sedeat.
And in reading of many books is distraction. Accordingly, since you cannot read all the books which you may possess, it is enough to possess only as many books as you can read. "But," you reply, "I wish to dip first into one book and then into another." I tell you that it is the sign of an over-nice appetite to toy with many dishes; for when they are manifold and varied, they cloy but do not nourish. So you should always read standard authors; and when you crave a change, fall back upon those whom you read before.
Distringit librorum multitudo; itaque cum legere non possis quantum habueris, satis est habere quantum legas. 'Sed modo' inquis 'hunc librum evolvere volo, modo illum.' Fastidientis stomachi est multa degustare; quae ubi varia sunt et diversa, inquinant non alunt. Probatos itaque semper lege, et si quando ad alios deverti libuerit, ad priores redi.
7.9.15 (to Fuscus Salinator, tr. Betty Radice):
Remember to make a careful selection from representative authors in each subject, for the saying is that a man should be deeply, not widely, read.
Tu memineris sui cuiusque generis auctores diligenter eligere. Aiunt enim multum legendum esse, non multa.
John Aubrey, Thomas Hobbes
, in Brief Lives
He had very few bookes. I never sawe (nor Sir William Petty) above halfe a dozen about him in his chamber. Homer and Virgil were commonly on his table; sometimes Xenophon, or some probable historie, and Greek Testament, or so.
He had read much, if one considers his long life; but his contemplation was much more then his reading. He was wont to say that if he had read as much as other men, he should have knowne no more then other men.
Gustave Flaubert, letter to Louise Colet (February 17, 1853):
How learned one would be, if one knew well only five or six books! John Frederick Peto, Old Companions
Comme l'on serait savant si l'on connaissait bien seulment cinq à six livres!