Free Kindle Books: A Guide, by Lauren Indvik

1. “Project Gutenberg: 33,000 free e-books, including all of the classics, available in Kindle, HTML and simple text formats.
2. Google E-Bookstore: The free section is filled with thousands of free, scanned copies of books, available in Kindle-friendly PDF formats.
3. Internet Archive: Millions of primarily rare, out-of-print works in multiple languages and formats (including Kindle), especially useful for academic work.
4. Open Library: 20 million user-contributed items in multiple editions and formats (including Kindle).
5. ManyBooks.net: Nearly 30,000 titles, many of which have been pulled from Project Gutenberg. Has a good collection of little-known Creative Commons works.
6. LibriVox: Thousands of free audiobooks.”