The owl-eyed, bespectacled man whom Nick and Jordan encounter in Gatsby's library gives us one of the first hints that Gatsby is a fraud. Owls, of course, have keen vision in the dark. Like the owl, this seemingly insignificant man, too, will see things others have taken for granted.
The books on the shelf reveal important clues when inspected. The man expresses surprise to the pair that the books on the shelves are indeed real, not fake, as he had expected.
"See!" he cried triumphantly. "It's a bona fide piece of printed matter. It fooled me. This fella's a regular Belasco. It's a triumph. What thoroughness! What realism! Knew when to stop, too-didn't cut the pages. But what do you want? What do you expect?"
There is a lot going on in this comment from a drunkard. For one thing, he refers to Gatsby as a "regular Belasco." Belasco was supposedly a sorcerer in the 13th century, whose exploits were detailed by Dante. The drunk is also pointing out just how far Gatsby goes to recreate realism...far, but not too far. The book's "uncut" pages means that the text has literally not been cut into individual pages, making the books real, but impossible to read.
Like the party atmosphere Gatsby manufactures, close inspection reveals it all to be a unsustainable spell. Nick and Jordan, however, are not ready to be disenchanted.