For, to judge by the words of the phrase, 'My son, beware of making many books,' two things appear to be indicated by it: first, that we ought not to possess many books, and then that we ought not to compose many books. If the first is not the meaning the second must be, and if the second is the meaning the first does not necessarily follow. In either case we appear to be told that we ought not to make many books. I might take my stand on this dictum which now confronts us, and send you the text as an excuse, and I might appeal in support of this position to the fact that not even the saints found leisure to compose many books; and thus I might cry off from the bargain we made with each other, and give up writing what I was to send to you. You, on your side, would no doubt feel the force of the text I have cited, and might, for the future, excuse me. But we must treat Scripture conscientiously, and must not congratulate ourselves because we see the primary meaning of a text, that we understand it altogether. I do not, therefore, shrink from bringing forward what excuse I think I am able to offer for myself, and to point out the arguments, which you would certainly use against me, if I acted contrary to our agreement. And in the first place, the Sacred History seems to agree with the text in question, inasmuch as none of the saints composed several works, or set forth his views in a number of books. I will take up this point: when I proceed to write a number of books, the critic will remind me that even such a one as Moses left behind him only five books.
Quote taken from Commentary on the Gospel of John V.2