The "Monstrous Regiment" in question is made up of a vampire, a troll, Igor, a collection of misfits and a young woman who shoves a pair of socks down her pants to join the army. Here you have the characteristically charming novel by Terry Pratchett.
Polly becomes Private Oliver Perks, who is on a quest to find her older brother, who's recently MIA in one of the innumerable wars the tiny nation of Borogravia has a habit of starting with its neighbors. This peevish tendency has all but expended Borogravia's ranks of cannon fodder. Whether Sergeant Jackrum knows her secret or not, he can't afford to be choosy as Perks and her/his comrades are among the last able-bodied recruits left in Borogravia. This collection of misfits includes the aforementioned vampire (reformed and off the blood, thank you), troll, and macabre Igor, who is only too happy to sew you a new leg if you aren't too particular about previous ownership. Off to war, Polly/Oliver learns that having a pair of, um, socks is a good way to open up doors in this man's army.
For those who haven't made this underrated author's acquaintance, "Monstrous Regiment" is as good a place to start as any. Readers will encounter Pratchett's subtle and disarming wit, his trademark footnoted asides along with a not-too-shabby tale of honor, courage and duty in the face of absurd circumstances.