Discworld 31: Monstrous Regiment

It began as a sudden strange fancy...
Polly Perks had to become a boy in a hurry. Cutting off her hair and wearing trousers was easy. Learning to fart and belch in public and walk like an ape took more time...
And now she's enlisted in the army, and searching for her lost brother.
But there's a war on. There's always a war on. And Polly and her fellow recruits are suddenly in the thick of it, without any training, and the enemy is hunting them.
All they have on their side is the most artful sergeant in the army and a vampire with a lust for coffee. Well... They have the Secret. And as they take the war to the heart of the enemy, they have to use all the resources of... the Monstrous Regiment.