Monstrous Regiment : Discworld Novel 31

A sudden burst of nationalism has swept over Polly Perks causing her to cut off her hair, don her brother’s clothes and join the local regiment so she can fight for her country. But there’s only one problem -- she has no idea who she’ll be fighting or what she is really fighting for. And why do they want her to have a rolled-up pair of socks anyway? War teaches you a lot, she finds, when it turns out that you joined -- the Monstrous Regiment.