This is the latest entry in Pratchett's four-book sub-series about a young witch, Tiffany Aching, as she grows up and learns, appropriately enough for her trade, to be a wise woman. (There are upwards of thirty or forty "Discworld" books total, which cluster into subgroups around individual characters). While you could certainly start here if you wanted to, new readers might find it more rewarding to begin with The Wee Free Men, the first in Tiffany's sub-series, followed by A Hat Full of Sky and then Wintersmith, before proceeding to this one.
This is billed as a children's / young adult book, although little sets it apart from Pratchett's other fantasy except for some (very) slight bowdlerizations; primarily, this is a young adult book because the heroine is a young adult, and it deals with issues that young adults have to deal with. Like the Harry Potter books, the content and tone of the Tiffany series have been maturing ever so slightly with each book to match the advancing maturity of the protagonist, and while this one's still suitable for younger readers, it definitely contains a few jokes likely to fly over their heads (at least unless some other source has educated them). Tiffany herself is portrayed as very mature for her age - a portrayal deliberate on Pratchett's part, I believe, as Tiffany is exactly as mature as most people that age tend to think they are, and almost as mature as she herself wants to be.
Each volume in the series sets Tiffany a particular problem to resolve; here the problem (s) are innuendo, rumour, gossip, romantic rivalries, and pointless mob hatred, things that many if not most teenage girls will identify with (even if in Tiffany's case the "witch hunt" she has to deal with is somewhat more literal). Tiffany's prior romance with Roland, the son of the local Baron, has clearly ended, and Roland's is about to marry Letitia, the daughter of a (very obnoxious) Duchess; meanwhile, some of the residents of the Chalk are stirring up hatred and accusations against Tiffany, and stalking her is the Cunning Man, a personification of suspicion, envious rage, hatred, mob violence, and the witch hunt.
Pratchett's typical mastery is still present here, his wit and his wisdom; the only real sign of his advancing illness is that there's a sense, especially in the novel's conclusion, that this may be the final Tiffany Aching novel and the final novel of his Witches series (if only because it features a cameo appearance from a character we haven't seen since the very first Witches-series novel, Equal Rites, first published twenty-plus years ago). All in all, it's an excellent book, fully as good as anything else he's written, and a book that will definitely please fans of the series and new readers alike.