Discworld. Innkeeper's daughter cuts off her hair and joins the army to save her brother and her inheritance and, eventually, her entire crazy country.
Oh, yes, I loved this one. I mean, Discworld does cross-dressing, of course it's awesome. It's also scary in places, and sewn with a few nasty little bites of what people can do to each other – can do to young girls, mostly. Not the deepest book he ever wrote, partly because Polly is kind of his standard-issue girl protagonist: she's whip-smart, determined, and clear-eyed. But he defaults to that type because that type works, so there's that.
And in the background there's Vimes, stomping around being cranky because when two drunks fight, you just bang their heads together until they quit, so what's he supposed to do here, bang two countries together? Well, naturally.