These are sites and tools I use when I’m editing book information on Shelfari, LibraryThing, and Goodreads, but they can be useful for other book-related purposes.

General
Wikipedia
Yahoo! Search
Yahoo! Image Search

Bookstores
Amazon
Amazon UK
Amazon Germany
Amazon Canada
Amazon Japan
Alibris

Libraries
Library of Congress Catalog
WorldCat

Literary Awards, Other Book Lists
Book Prize Info
Wikipedia: Literary Awards by Country
Wikipedia: Publishers Weekly Bestselling Novels in the US, 1900-Present

Language
Babelfish
LEO German-English Dictionary
WWWJDIC: Online Japanese Dictionary Service
Poltran, Polish-English translator
Dictionary.com (English)

Shelfari
Shelfari
Shelfari Librarians & Editors Group
Editing Guidelines, Policies, and Helpful Hints Group
Shelfari Questions & Help
Shelfari Blog

LibraryThing
LibraryThing
CommonKnowledge Help Page
Talk about LibraryThing Group
New Features Group
Hacking LibraryThing Group
Common Knowledge, WikiThing, HelpThing Group

GoodReads
Goodreads
Goodreads Librarians Group
Librarian Manual