What does Owl Eyes discover, and why is this shocking in The Great Gatsby?

Owl Eyes discovers that the books in Gatsby's library are real; he is surprised because he knows that the parties are contrived and the guests are "brought."

In Chapter Three, Gatsby has a replicated hôtel de ville for his mansion. There on the imitative West Egg, he throws lavish parties every couple of weeks. A corps of caterers arrive to erect an outdoor pavilion and set up buffet tables that hold all sorts of attractive...

Owl Eyes discovers that the books in Gatsby's library are real; he is surprised because he knows that the parties are contrived and the guests are "brought."

In Chapter Three, Gatsby has a replicated hôtel de ville for his mansion. There on the imitative West Egg, he throws lavish parties every couple of weeks. A corps of caterers arrive to erect an outdoor pavilion and set up buffet tables that hold all sorts of attractive hors d'oeuvre and appetizing fowl and other meats. There is even an authentic bar with a brass rail replete with liquors and cordials that is set up for the guests.

During the party Nick and Jordan seek Gatsby in his mansion, and they enter the library where "a stout middle-aged man with huge owl-eyed spectacles" is seated upon a table, staring at the shelves that hold leather-bound books. Assuming that Nick and Jordan think Gatsby's pretentious parties and his home are all for show as he does, the man asks them what they think about the books. Rather nonplussed, they look at Owl Eyes.

"Absolutely real--have pages and everything. I thought they'd be nice durable cardboard....Here! Lemme show you."

Hurrying to a shelf, he takes down Volume One of the "Stoddard Lectures" [This is the same book that Tom Buchanan has on his shelf in Chapter One.]

"This fella's a regular Belasco [famous playwright and director]. 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?"

Owl Eyes, whose appearance resembles the billboard with Doctor T. J. Eckleburg who wears "a pair of enormous yellow spectacles," observes more than than other people do. For, he obviously suspects Gatsby of assuming a role since he and other guests have been "brought" to the party.