I saw it mentioned in Lambda the Ultimate yesterday. Google has added the Google Print (Beta) service which offers scanned books online. You can browse the book online, search for words within the book, get the metadata about the book, or buy the book from online booksellers.
The books come from publishers and libraries.
Just do a regular Google search. If there is a book that matches, it will be shown near the top of the search results, the same way Google Desktop Search results are displayed.
When I tried it, only one book is ever shown. And you pretty much have to type in the exact title to get it. I tried "War and Peace", "The Wealth of Nations", and "Tom Sawyer."