Inside, of course, there are rare books and maps and exhibits, which are worth the trip. There are also events held here--I strongly suggest attending one because the content is superb (whether lecture or string quartet) and the southeast room (don't remember the name) that many are held in is sumptuous.
The book/gift store is small but jam-packed with books, stationery, book-related gadgets....really too many things to count. You need at least a 1/2 hour, if not hour, in this store. And make sure it's a payday.