Ready Reference: Books in Print & Publishers

Amazon.com: Search 2.5 million titles by author, title, subject, keyword, ISBN, or publication date.
BarnesandNoble.com: Search over 1 million titles by author, title, keyword, or category (e.g., Biography).
BestBuyBooks: Search over 25 bookstores simultaneously to find the lowest price.
Internet Book Shop: Contains details of over 912,000 titles available in the U.K. Approximately 90,000 titles have bibliographic information that is supplemented by full marketing information.

www.alapage.com: Search French books in print by title, author, publisher or browse by subject.

Advanced Book Exchange, Inc.: "The Internet's most popular service for buying and selling out-of-print, used, rare and antiquarian books."
alibris: Source for books, maps, photographs, autographs, documents, manuscripts, magazines, scholarly journals, and ephemera.
Bibliofind: "The largest booksearch site with 3 million old books, used books, rare books, out-of-print books, antiquarian books, offered by over 1000 booksellers around the world."
Bookfinder.com: Search engine that scans top book databases to find new, used, rare, and out-of-print books.