Quite easily America's best academic bookstore. The Seminary Co-Op offers over 100,000 new titles, practically all in the humanities and social sciences -- if it's in print, it's probably here. The comprehensive collection also includes large selections in sheet music, art/architecture, foreign languages, and humor beyond its vast collections in sociology, political science, economics, history, anthropology, religion, cultural studies (especially gender), environmental studies, semiotics, philosophy, textbooks, literature of all possible genres, collegiate interest materials (research guides, neighborhood maps...), and other fields I haven't even heard of.
Don't look for superstore fare here -- no children's books, multimedia, self-help or business books, magazines (except a few scholarly journals), bargain books, coffee, or comfy chairs. This is an academic bookstore, and they mean it.
The store occupies the spartan basement of the Chicago Theological Seminary (hence the name) and has rather low ceilings lined with pipes of various sorts. Books are arranged in a crazy labyrinth of nooks and corridors; consult the maps posted throughout the store or a (usually aloof but still helpful) staff member to find what you're looking for. The Co-Op sponsors a packed calendar of literary events, often held at its sister, 57th Street Books, or on University grounds. The stairway down into the bookstore hosts a surprising array of circulars and flyers advertising all manner of academic or literary events in addition to the standard city fare.
For $30 the Co-Op will sell you three shares of stock, entitling you to a lifetime 10% discount on all books (at all three Seminary bookstores, including 57th Street Books and Newberry Library), a share of any profits, and a say in its governance (alongside more than a few Nobel laureates).
Note: Unfortunately, the Seminary is NOT fully accessible.