Someone asked me to help her find a new book to read based on what she just read (and liked), so I thought I'd share this list (with credit to Lifehacker.com):
http://www.goodreads.com/ - most popular tool according to one website
http://www.whatshouldireadnext.com/ - Simple interface (type the title in slowly and select from a drop down list) - but no covers to browse until you click "more info" and land on a book's Amazon record.
http://www.mindthebook.com/ - selects a book based on a question, eg. "How to be happy?" - probably best for non-fiction
http://www.shelfari.com/ - kind of like facebook for readers, but the recommendations seem to be less relevant. No need to create an account to search, though.
http://www.lazylibrary.com/ - helps you find short books
http://www.authonomy.com/?from=bookarmy - I wasn't able to get the search function to work, so i don't know how good the results are
http://booklamp.org/ - promises to find books based on writing style, but I couldn't get it to pull up any books I'd read (even popular titles like "The Help", and "Catch-22")
http://www.whichbook.net/ - Instead of searching, you slide scales on the side eg. (Funny or Sad, Unusual or Conventional). You can also browse by Author, but that doesn't give you recommendations for other authors, just shows you where their books are on the scales. Fun to use, but not sure how useful.
http://www.yournextread.com/us/ - Easy to use interface, some of the recommendations are similar to Shelfari, so maybe the two websites are sharing data.
http://bookseer.com/ - Very simple interface, but you need to know the title and the author to search
http://www.librarything.com/ - Has been around a long time, free to join but there is a paid membership option. No searching without signing up.
http://getglue.com/ - Searching for members only. Free.
And of course, there is always http://www.amazon.com - probably the most powerful in terms of data, but what you will see is only the most popular books other people liked, and sometimes what is most popular is not particularly well written.
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[Addie Myers Fletcher, CCTSI Librarian]