While there is no way to restrict search results to downloadable books only, you can limit your results to books that are fully viewable by using the Advanced Search feature of Google Books. Just select the "Full view only" option, enter your search terms and click "Google Search."
Browse through key museum titles dating back to its founding in 1937. Click on a cover from the list, then click on Read Catalogue Online to view these excellent art books, complete with images. Also available in multiple formats from the Internet Archive.
Open Library is a project of the non-profit Internet Archive. The goal is one web page for every book ever published. Not every book listed has a free eBook version, but there are many available through Open Library and you can limit your search to eBooks.
Maintained at Stanford University, this site has an editorial board from Universities around the world. Topics that are currently covered, such as category theory and pantheism, have impressive in-depth information.