Steven Brust wrote an entire Firefly novel to tie in with the film Serenity. The bad news is that he wrote it on spec and they decided not to do any Firefly novels. The good news is that he’s released it online under a creative commons licence. You can download it here.
I had no intention of reading it right now and I’m a bit ambivalent about reading novels on a computer screen, but having got sucked into the prologue I think he’s captured the feel and voices of the series extremely well. I’m intrigued.
(I’ve never read any of Brust’s novels but Janet’s read quite a few and is a big fan of To Reign in Hell in particular.)