Mary Roach’s enormous cast of characters is the unluckiest in recent nonfiction. They are prodded in the genitals, riddled with bullets, dropped on land mines, tossed from planes, crucified like Jesus, gouged in the eyeballs, eaten by maggots, reincarnated as compost, ripped open like birthday presents, and gutted like tuna; some of them even stand by quietly as their butt cheeks are sliced off and served for dinner in China. They are, fortunately, corpses — a.k.a. ”the dead’s heroes” — and Stiff: The Curious Lives of Human Cadavers (Norton, $23.95) is Roach’s gross, educational, and unexpectedly sidesplitting history of their extra-mile sacrifices for humanity. Because she always draws a distinction between you and your smelly carcass (not the same person, she argues), Roach gets away with the cheerfully morbid smart-ass commentary that abounds throughout. She’s written one of the funniest and most unusual books of the year.