I use the book.app. Once installed, you need to install the winscp app to save your books into the proper directory. The correct directory to save in is riit/private/var/mobile/media/ebooks.
When you download books, be sure to download them in the ipod book app format. (you can also transfer them into HTML format to read them).
Don't have the URL in front of me, but search for some1guy for a source for books. I'm assuming you already have the app available, which is easily found with standard sources.
