Without having a list of the other "Dozen" books (I keep meaning to check them out)...
I *love* the Epic Creatures one mentioned above. That may have to be my request if I win. I love how it could integrate epics into a game, or supply the seed (allowing me to flesh out the finer details) for an entire campaign from its humble beginnings to its epic conclusion. And you'd get 12 of those in one book? I know I'd buy it.
Other suggestions...
"Dastardly Villans" Some of the archetypal villans from fiction and fantasy. From the gritty crime lord to the ever reviled evil step mother.
"Codes of Conduct" A collection of ethical (and some not-so-ethical) codes that players could choose for their characters to follow. Complete with circumstances that tend to foster such codes, or example societies that might place some importance on them.
"Weapons of Opportunity" Not sure how viable this idea is, but it could either be a listing of 12 individual items that could be used as weapons (or be otherwise beneficial in combat) or (if we are talking about a slightly larger product) it could be 12 environments (tavern, bedroom, prison, etc..) with a small list of items common to those environments that might be useful in combat.