Monstrous Regiment by Terry Pratchett
(Discworld series #31)
Polly Oliver is a young Borogravian whose older brother has gone missing in battle, so out of a sense of familial duty, she decides to join the army to see if she can find him. But that's not as easy as it sounds since Borogravia's antiquated laws prohibit women from joining the army. But with the help of a haircut, some boy's clothing, and a pair of strategically placed socks, she's able to enlist.
She's assigned to a regiment of other newly enlisted soldiers consisting of a vampire, a troll, and an Igor, each with their own secret--which likewise requires an extra pair of socks. This group of soldiers, who have no business being in the army, begin to stand out due to their bravery, cunning, and overall higher-than-normal intelligence level expected from soldiers.
Monstrous Regiment is another great addition to the Discworld series. This time Pratchett aims his satirical guns at the armed forces, and at war itself. Religion suffers a little collateral damage as well, but only where deserved. My only complaint with the book is that it's one of the Nightwatch books within the Discworld series, but Sam Vimes, a fantastic character in that group of books, appeared far too infrequently.
Monstrous Regiment would be a good introductory book to the series if you've never read one before. There's really no backstory to be aware of since very few recurring characters make an appearance.
★ ★ ★ ★ ☆