Project Gutenberg is a huge collection of 10,000 eBooks which you can use for free. You can see the website here and can use them. You can download them in
1. HTML
2. Plain Text
3. EPUB - EPUB (electronic publication) is a e-book standard, by the International Digital Publishing Forum (IDPF). EPUB files have the extension .epub
4. MOBI - MOBI (MobiPocket) is the format used by the the MobiPocket Reader. The MobiPocket Reader is no-cost proprietary software available for BlackBerry, PalmOS, Symbian, Windows and Windows Mobile. You can get it here
Here is an Application which will help you to access these books. This is an Open source software. get it here
I have wrote about this last year here