So we're thinking of playing the Serenity RPG. It seems that the main rulebook is Big Damn Heroes. Do we need any other books to get started? In particular, do we require something like the Cortex Role Playing Game System? Are there any other books/accessories that would be absolutely necessary to play?
The Serenity Role Playing Game book is a complete game in and of itself. It's the "core rules" for the Serenity game.
You don't need the Cortex System book, and in fact, due to differences between the two, it's only going to confuse things.
You probably should get the Big Damn Heroes Handbook. The rules and setting expansion it provides is worth the money. Hell, the extra 'verse chinese is almost worth it...
It really helps to have:
The line, as best I can tell:
And it looks like the line's been canceled, by the way. It's no longer available in PDF from Drive Thru, nor mentioned on MargaretWeis.com online store. (and it was available in PDF in april 2011.)
There is a good bit of fan-written material, as well, mostly ships and adventures.
As a bit of an aside: