Steven Brust has written a novel set in the Firefly universe, and he released it under a Creative Commons license. It's available in PDF and Word; so should be easily convertible to other formats.
I haven't read it yet, so I can't recommend it yet... But it's a Firefly novel, written by Steven Brust, how shiny is that?
You can find it here:
http://dreamcafe.com/firefly.html.