The Xidan bookstore has a really great range of books. I've gotten pretty obscure Chinese literature there as well as a modern philosophy book that I enjoyed a lot. Friends of mine have also gotten really great dictionaries (electronic ones too) upstairs, which is a great resource for living in China. I think the one spot that's lacking at Xidan is the contemporary literature. I think you're much more likely to find New York Times Bestseller type books at Bookworm, as Xidan doesn't really specialize in that stuff. One way to make sure it to ask the staff. They're English is generally pretty good and they're super helpful in finding things. Also, they won't try to sell you things you haven't expressed interest in which is nice. Even if you don't end up buying anything, this place is so large and classic that it's worth visiting as a cultural site as well.