These are a few of the more interesting sites I have found for free ebooks for the Kindle and other devices. Many of them only have PDF files. The OpenLibrary also features books that can be “checked out” like a library which will deactivate after a given amount of time.
Free Ebooks:
ManyBooks: http://manybooks.net/titles.php
Project Gutenburg: http://www.gutenberg.org/ebooks/
OpenLibrary.org http://openlibrary.org/
Archive.org Texts: http://archive.org/details/texts
Amazon also has a collection of free books, largely classics: www.amazon.com/b?ie=UTF8&node=2245146011
Or, you can search any subject of Kindle Books on Amazon and simply sort by prices lowest to high to see free books in a wide variety of subjects.
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