Monstrous Regiment - Terry Pratchett

'That's the trouble about the good guys and the bad guys! They're all guys!' In the small yet aggressive country of Borogravia, there are strict rules citizens must follow. For a start, women belong in the kitchen - not in jobs, pubs, or indeed trousers. And certainly not on the front line. Polly Perks has to become a boy in a hurry if she wants to find her missing brother in the army. Cutting off her hair and wearing the trousers is easy. Going to war however, is not. Polly and her fellow raw recruits are suddenly in the thick of a losing battle. All they have on their side is the most artful sergeant in the army and a vampire with a lust for coffee. It's time to make a stand. The Discworld novels can be read in any order but Monstrous Regiment is a standalone.