Richard Elliott has shared some real peaches over the years in his eLearning Watchfor. A couple that I have been particularly taken with are The Open Library and The Open Academic Textbook Catalogue.
The Open Library
This amazing resource offers a searchable (editable) database, with, once you find a book you want opportunities to read, borrow, and/or buy. Some books only have the buy option, but many have a download option in a wide range of file types (or you send to your Kindle).
Richard described the library as follows: "The Open Library. Is an ever-growing repository of books of all types and subject. It is an open project: the software is open, the data are open, the documentation is open, and contributions are welcome. Already the site claims to have over 1,000,000 free e-book titles. Plenty you will recognise and maybe read or direct your students to."
If you are studying, or just interested in learning this is the place to go. The easily searchable site offers textbooks to print (at a cost, but way less than it would cost to buy from a shop), as well as in the following formats .PDF, audiobook, EPUB, mobi, and 'read online' (usually free). As Richard says, "Always good to see efforts being made to ensure educational resources can be obtained freely and with minimum cost or no cost at all. This initiative is great and is open to any faculty world wide".