The Grolier Club

I love all the suggestions made already, but I'd also suggest the Grolier Club, a private club for book collectors and bibliophiles. HOWEVER, the first two floors are open to the public and they have fabulous exhibitions--June is "Risings: The Irish Literary Revival and the Making of a Nation" and "New Members Collect," a sampling of the collections of new Grolier members. All of these are wildly fascinating! You can also make an appointment to see their beautiful library.