I quite liked this one - I think as a general rule I quite like the Discworld books with female main characters. He's quite the feminist our Terry and does a good job of describing a woman rebelling against her traditional lot.
In Monstrous Regiment we meet Polly who joins the Borogravian army, disguised as a boy, to try to find her brother Paul. Borogravia is a country forever picking fights with its neighbours and suffering under the increasingly arbitrary commandments of their god, Nuggan, who has condemned a million things. This includes women in the army, but also red hair and garlic.
Polly's new regiment includes a vampire, a troll and an Igor, apart from a selection of humans - one of whom is extremely religious and claims to speak to the Duchess (the by now almost mythical leader who acts as a go-between to Nuggan). Gradually Polly begins to suspect that she is not the only woman (this is not a spoiler, given the title of the book). They also discover they aren't doing so badly on the fighting front, and that someone is clearly frightened of them, and someone else looking out for them.
There are many very funny and clever bits in this Discworld novel, from the satirical portrayal of military brass and religion to the character spins - Pratchett's trolls and vampires are in their own league. My one quibble is that I didn't really understand the end, but I was in a rush then and skimmed it (not expecting him to want to Say Something). I may re-read it.