Book Search Up & Running

Lots of out-of-copyright books can now be downloaded in PDF format for free at Google Book Search.

Also:

• You can view snippets of books, limited preview, full preview and more
• They have a partner program which allows you to promote your books on Google for free and add a purchase link which must go to your site and can not be an affiliate link.

On the downside, the number of available books is quite low, definitely not comprehensive. Similar functionality can be foudn at Amazon & Gutenberg (which has 17,000 free books)