What does the owl-eyed man find extraordinary about Gatsby's library?

The books are real, not just cardboard boxes for show. Moreover, they have not been "cut."

In 1920's when a book was bound the pages were still stuck together and had to be cut apart to read them, leaving jagged edges. The owl-eyed man notices that Gatsby presents the books as pretense but does not try to fool anyone that he has actually read them. He knows not to push the fakery too hard.

This is also a way Gatsby shows of his wealth, to win back Daisy.

"Absolutely real -have pages and everything......Knew when to stop too, -didn't cut the pages...."pg 51-52