Monstrous Regiment: Discworld Novel 31 (Discworld Novels)

 As her country fights an insane and mostly unwinnable war, Polly Perks signs up to the army to find her brother Paul. As dressing as the opposite sex is one of a multitude of insane religious rules in her country, she employs short hair, trousers, burping and socks in her pants to ensure she enlists as a man. As she signs up alongside a troll, a vampire and a zealot among others, she rapidly learns she’s not the only woman in the military. It’s Pratchett so you know it’s awesome, but it’s a great ride through the absurdities of war, nationalism, sexism, and religion. But it’s also a fun comic take on the very very very long history of women cross-dressing to join the military (many of whom had distinguished service before they were discovered).