This is a very brief list of useful data sources if people want stuff to play with on the day. I (James Bridle) have played around with most of them, and investigated lots of others, so if there's a particular thing you're after, do ask and maybe I can think of something.

• LibraryThing API: ISBNs, titles and languages. (Requires LT membership, developer key.)
• LibraryThing Web Services: for querying LT's 'common knowledge' data (ditto).
• LibraryThing Covers: 1m+ covers (ditto)
• Amazon Associates Web Service: covers, bookdata, reviews (requires AWS account).
• xISBN: Worldcat's ISBN service
• isbndb - another ISBN database, also includes price comparison.
• Google Book Search API: book previews, text search and bookdata.
• BookMooch API - find books for free.
• DBpedia: for querying Wikipedia.
• Bkkeepr API: very limited access to user's reading data. Ask if you want more!

There's also lots of texts available for free at the following locations:

• Gutenberg: free book texts in HTML and plain text
• FeedBooks: free book texts in ePub, Mobipocket/Kindle, PDF , Sony Reader and iLiad formats.
• DailyLit: receive a book via RSS or email at the rate of a chapter a day (or according to your own schedule); see also: Podiobooks, which does something similar for audio books.

Feel free to add to this list...