AbeBooks.  Includes contemporary as well as out-of-print books (and some entertaining blog entries).     
Anonymous photographer. Book stalls in the Passage Jouffroy, ca. 1905.
    "Compare prices on over 150 million books for sale."

Cellar Stories Bookstore: Largest used and rare bookstore in Rhode Island.

20th Century Art Archives. "Specialist dealers in archives, rare and elusive books, documents and reference works on 20th Century Art and Artists."