Faber initially explains the value of books to Montag as records of humankind's history: they preserve both the greatest thinking of all time and, importantly, the dire mistakes men have made. They are instructive, and they are cautionary: they warn us of what not to do again. Therefore, books help prevent us from making the same mistakes over and over.
But Montag also learns that it is not the books themselves that are important. It is the knowledge that is inside them that matters. Having books around means nothing if nobody reads them, thinks about them, and argues about their ideas with other people.
At the end of the novel, Montag even learns that Granger's men are book burners too. They memorize books and then destroy them so that they can avoid arrest. They can do this because they realize the value of the books are in their contents, not their physical being.