Google Books offers links to booksellers from whom you can buy the books you find. In the case of out of print books, the links go to used bookstores. Booksellers don't pay us to include their links, and we don't make money when you buy books from them (unless you purchase a Google eBook edition).
Some books are available as Google eBooks, in which case we have an agreement to split the revenue with the appropriate rightsholder. If you purchase the book from one of our retailer partners instead of directly from Google, the retailer will also receive a share of the revenue.
We do earn revenue from user clicks on the contextually targeted ads that appear, with publisher permission, on some books that are part of the Partner Program, and we share this ad revenue with those partners.