Monstrous Regiment by Terry Pratchett (2003, Hardcover)

War has come to Discworld . . . again.

And, to no one's great surprise, the conflict centers onthe small, insufferably arrogant, strictly fundamentalist duchy of Borogravia, which has long prided itself on itsability to beat up on its neighbors for even the tiniestimagined slight. This time, however, it's Borogravia that's getting its long-overdue comeuppance, which has left thecountry severely drained of young men.

Ever since her brother Paul marched off to battlea year ago, Polly Perks has been running The Duchess,her family's inn -- even though the revered nationaldeity Nuggan has decreed that female ownership of a business is an Abomination (with, among others, oysters, rocks, and the color blue). To keep The Duchess in the family, Polly must find her missing sibling. So she cuts off her hair, dons masculine garb, and sets out to join him in this man's army.

Despite her rapid mastery of belching, scratching, and other macho habits (and aided by a well-placed pair of socks), Polly is afraid that someone will immediately see through her disguise; a fear that proves groundless when the recruiting officer, the legendary and seemingly ageless Sergeant Jackrum, accepts her without question. Orperhaps the sergeant is simply too desperate for freshcannon fodder to discriminate -- which would explain why a vampire, a troll, a zombie, a religious fanatic, and two uncommonly close "friends" are also eagerly welcomed into the fighting fold. But marching off with little (read: no) training, Polly (now called "Oliver") finds herself wondering about the myriad peculiaritiesof her new brothers-in-arms. It would appear that Polly "Ozzer" Perks is not the only grunt with a secret. There is no time to dwell on such matters, however.Duty calls. The battlefield beckons. There's a tide to be turned.

And sometimes -- in war as in everything else -- the best man for the job is a woman.