One of Pratchett's weaker books, in my opinion. Starring a completely new cast of characters, with a few minor cameos from a few older ones, the book starts where a girl joins the military a la Mulan.

Eventually, she finds out that she is not the only one in her squad who is secretly a female.

Then, one by one, it turns out that everyone in her squad is female. This is stretching my suspension of disbelief near its breaking point, but I'm willing to go along with it for now.

But at the end of the book, almost every single major character is female. The only major character I can think of that isn't a lady is a Lieutenant whose name is Blouse. Even the characters who had been made out to be obviously male at the beginning of the book turn out to be chicks.

Now, I understand one or two. Maybe even, by coincidence, an entire squad. But near the end, it no longer has any impact when someone's a girl, yet each revelation is as drawn-out as if it were the first, and each one is made out to be as if it were a surprise. By the final third of the book, my suspension of disbelief was completely shattered.

You would think that someone would have noticed, since apparently this had been going on for years. Girls don't exactly have deep voices. They stuff their pants with socks to give the illusion that something is down below the belt, but it would seem that no girl in the entire country is gifted in the upper torso department.

However, despite its flaws, it is a Pratchett book. And like most Pratchett books, it is a decent, funny book. The analogy that one can never have a bad pizza comes to mind.
But, sadly, this is probably one of his lesser works. If you just want Pratchett, try Mort, Reaper Man, or Hogfather (I may be a bit biased in what stories I recommend). This book should only be read if you're running out of Pratchett books to read.