I think the fact that he points out the reality of Gatsby's books, the fact that they aren't just facades is very significant. The era of the twenties is very indicative of people who put on a false exterior and are really quite "empty" on the inside. Gatsby's parties are filled with people who are quite empty and are only there for a good time, not because of their credible relationship with Gatsby. I think the man, Owl-Eyes (with a name suspecting he can certainly see into things), who was actually brought to the party and has little concept of Gatsby himself thinks that since Gatsby has the full library pages instead of just fake books, it makes Gatsby's identity somehow more real. But who knows if Gatsby really reads them, this could in itself be another facade. Thus, I believe this entire scene to be an exploration of identity and the truth of character.
Books are often a great metaphor in our world, have you heard, "Don't judge a book by its cover"? Think about it.
I think this small scene from early in the novel is also important later when Owl Eyes is one of the only people to attend Gatsby's funeral. Perhaps he comes because he "knows" something about Gatsby. In his estimation there was a "truth" about Gatsby in that he had REAL books -- he appreciates that Gatsby isn't just putting up the facade like others probably were. That real connection compels him to honor Gatsby at by attending his funeral.
The discovery by Owl-Eyes that Gatsby has leather-bound real books impresses this observant man who has hitherto thought of Gatsby as very superficial. This moment is the beginning of a certain respect for Gatsby, and because of this respect for the genuineness of Gatsby, Owl Eyes comes to his funeral.