Online Bookshops

Advanced Book Exchange Search ABE for used, out of print, rare, antiquarian and hard to find book
Alibris - The Ultimate Source for OP and Rare Books
All Direct Books
Alphabet Street
Amazon.co.uk The UK site
Amazon.com : Books, Music & More! The US site
Barnes & Noble - The World's Largest Bookseller Online
Bibliofind - Antiquarian Books, Old Books, Used Books, Rare books,bookselling,bookseller
Blackwell's Online Bookshop
Bookpool : Discounted Technical Books
Books on Line
Borders.com. The best selection in the world
Evenbetter.com, Comparison Shopping - was Deal Pilot Compare prices from online shops
Heffers
Heritage Archaeological Books
Internet Bookshop [W.H.Smith on-line]
Penguin Books
Powell's Books - New, Used, and Out of Print
Waterstone's Bookstore

Or for out of copyright electronic books :-
PROJECT GUTENBERG