Actually, Grahame-Smith is not the only author. Rather, he collaborated with Ms. Austen, despite the fact that--much like many of the characters in the book--she's dead. I'll let the official press release do the explaining:
Pride and Prejudice and Zombies features the original text of Jane Austen's beloved novel with all-new scenes of bone-crunching zombie action. As our story opens, a mysterious plague has fallen upon the quiet English village of Meryton—and the dead are returning to life! Feisty heroine Elizabeth Bennet is determined to wipe out the zombie menace, but she's soon distracted by the arrival of the haughty and arrogant Mr. Darcy. What ensues is a delightful comedy of manners with plenty of civilized sparring between the two young lovers—and even more violent sparring on the blood-soaked battlefield as Elizabeth wages war against hordes of flesh-eating undead.
With any luck, this will spur a trend of classics of English literature being interpolated with zombie horror. Bleak House of the Dead is soon to follow, I'm sure.