You are correct that the reference is a little obscure -- if my memory serves it's Plato's Phaedrus.

Plato argues, in a book, in the voice of his mentor Socrates, that books will destroy knowledge. And the reason is precisely to do with the thing that Socrates is famous for, his Socratic Method of asking his students (or rival politicians or anyone) questions about what they claimed to know, engaging them in a real discussion where we come to things that we agree about and then build that agreed knowledge-base incrementally. For Socrates, a book could never provide that experience; you cannot ask the book a question.