AbeBooks. Includes contemporary as well as out-of-print books (and some entertaining blog entries).
Bookfinder.com. "Compare prices on over 150 million books for sale."
Cellar Stories Bookstore: Largest used and rare bookstore in Rhode Island.
Studio Books: Rare, Out of Print.
20th Century Art Archives. "Specialist dealers in archives, rare and elusive books, documents and reference works on 20th Century Art and Artists."