Every one of his books, this one included, shows a great sense of humor and a sense of the absurd. As Mr. Pratchett began writing as an antidote to "bad" fantasy (see his website), he can't help but poke fun at some of the worst out there. In this book, he thumbs his nose at traditional fairy tales.
You know the ones...where the princes are all handsome, the princesses swoon (and are invariably blonde) and witches are all evil. Things are never that cut and dried in real life or in The Wee Free Men.
Tiffany (an unlikely name for a witch), lives on The Chalk and is 9 years old. She just happens to be around when the very fabric of the world is in danger from an invasion by the Queen of the Fairyworld. Since there's no one else to take care of the problem, Tiffany does.
Well, she also fights on because the Queen stole her baby brother. She's not all that fond of him, since he's always sticky and a pain, but nevertheless, it just isn't right for the Queen to think she can just go around stealing babies.
The Wee Free Men (a.k.a. The Nac Mac Feegle) help her out, as does a cursed toad. Here's where the funniest scenes in the book play out. I challenge anyone to keep a grin off their face while reading about the exploits and derring-do of the Wee Free Men.
I have just one caution about this book. The heroine is only 9. Typically, kids and teens like to read books about characters that are a few years older than them, so some older kids may not feel that this book is for them. However, Tiffany doesn't feel like a 9 year old and this book is perfectly appropriate for anyone that age or older (though there are a few scenes that younger readers will simply pass over as funny, when they actually have some, ahem, deeper meaning).
Any fan of Pratchett's will also enjoy this book. Shoot, anyone with a sense of humor should enjoy this book. I personally love Tiffany's matter-of-fact attitude about everything from birthing sheep to wacking Jenny Green Teeth in the head with a frying pan.