I know that it allows you to put any books you like on it if you jailbreak it, but I'm not really sure where else I can buy ebooks and the money goes to the author.
Also I actually quite like the default reading experience (I have a paperwhite) and jailbroken stuff often has less polish.
You can also add books by setting up an email address for the kindle on the amazon site and then sending PDFs to it directly. Some other formats also work.
Only problem is finding a source for the books.