One of the world's great bookstores. They have a decent coffee shop; they sell new and used books; they fill several stories of a full city block (and still need *several* annexes); AND they shelve new and used books together!
Special note: If you're visiting from out of town, and go overboard (easy to do), they will even ship the books you buy there!
They call it The City of Books for a reason. This is the largest new and used bookstore in the world. It's got a great coffeeshop (bring books in to read while you shop!) and a large rare book collection.