Yes you can. The format is prc. You can also get the mobipocket publisher and use it (on your desktop) to convert html files (or books) to prc. There is another desktop program I use to convert .lit books to html (the ones that aren't locked), called convertlit.
Some good sites are baen books, fictionwise, manybooks, just do a google search and you'll find some.
http://www.aximsite.com/boards/showthread.php?t=64210
Check out the thread above. :approve: