This is the sixth book in the Discworld series, but a good place to start if you haven't read any others. It introduces the small mountaintop kingdom of Lancre and the witches, wonderful characters who will continue to feature in several subsequent Discworld books.
Like all Pratchett's books there are levels on levels on levels of jokes, and those familiar with Shakespeare's plays (especially Macbeth) will find lots of in-jokes and references, while those who are not will not enjoy the book any the less for not catching every allusion.
The book introduces the Discworld's version of witchcraft, while taking a sharp and irreverent look at theatre, royalty, fairy tales, human nature, and the nature of destiny itself, as always in an extremely irreverent and hilarious way.