Mostly books on power and crime.

He does seem very into Organized Crime.

Don Quixote, Don Juan, and Invisible Man were surprises.

 The novels were from the "more interesting" bookshelf of romance novelist Jennifer Egan.  How did you do on the reading comprehension part of the GRE?  ;)

Gladwell explains his interest in power:

I’m in the middle of writing my new book, which is about power. I’m very interested in the strategies we use to keep people who are powerless in check. And the ways in which the powerless fight back. So I started reading about crime. I’ve probably acquired 150 books for this project. I haven’t read all of them, and I won’t. Some of them I’ll just look at. But that’s the fun part. It’s an excuse to go on Amazon. The problem is, of course, that eventually you have to stop yourself. Otherwise you’ll collect books forever. But these books are markers for the ideas that I’m interested in. That’s why it’s so important to have physical books. When I see my bookshelf expanding, it gives me the illusion that my brain is expanding, too.

Source: http://www.theglobeandmail.com/arts/books-and-media/whats-on-junot-diazs-bookshelf-and-malcolm-gladwells-and/article5370351/

 Can't wait. Seems like will be his best -- and long-term, most interesting to me -- book yet.