If you haven't tried Project Gutenberg yet, you need to try it... With HTML, PDF, EPUB, and even TXT formats you are bound to find a format that will work for you..
Most if not all the books listed are public domain and are a wonderful option for the tight budget that so many of us are on.
When I was blessed with a Nook during e-book week earlier this year I started uploading all kinds of pdf files to my new toy... Tonight I realized if I uploaded EPUB format I could use all the features of the NOOK. I was even more excited to find several books I am currently reading (The Desire of Ages, The Great Controversy, and several others) all available in EPUB format.. now I can read the books and make notes without carry a huge book bag everywhere.