I use MobiPocket every day to read books and magazines on my BlackBerry. Its great to always have a book in the bathroom too. The thing is that e-books get a bit expensive if you read as fast as I do so I found two great resources for free books.
www.ManyBooks.net – This site was first was brought to my attention by Josep. It is a great resource where you can find public domain books. The best part is that you can download them directly in MobiPocket format.
www.Gutenberg.org – This is the project Gutenberg website with over 25,000 free books to download. You will have to convert the books into MobiPocket format using the free desktop application.