The Rare Book Department at the Free Library of Philadelphia is a specialist collection with enough treasures to interest more than book collectors. Highlights include a Gutenberg Bible, medieval manuscripts, children's book illustrations, and the preserved body of Charles Dickens's raven Grip, which helped inspire Poe's "The Raven."
Public tours are the best way to see the collection. Check the current schedule before going, as access is more limited than in a normal museum gallery.