Harry is right, nicely converted books are better, but I have also downloaded and read plain text files from Project Gutenberg. They're usable and readable if you can ignore the lack of formatting. They're free, putting them on the Kindle is a very small investment in time, and the worst that can happen is that you won't like it, you'll delete it and start over.
I would also strongly second AnemicOak's suggestion to use the libraries here. I've been impressed with how good a job folks here do with the texts--often better than Amazon, I hate to say (but I must).