Hosted by the University of Pennsylvania Library, the Online Books Page Web site "facilitates access to books that are freely readable over the Internet...and aims to encourage the development of such online books, for the benefit and edification of all." The over 18,000 listings can be searched or browsed by author, title, and subject, of which every major physical science subject is represented. Each listing is linked to an outside Web site, making its availability unpredictable, but the shear number of books that are accessible make the Online Books Page a great resource.