Death takes a sort of holiday, so the Death of Rats goes looking for Death's granddaughter to fill the gap. Susan is off in boarding school, being an unusually practical teenager with no memory of her grandfather, when she gets the message. Meanwhile a bard, a troll, and a dwarf meet up and form a band and a magical guitar introduces the idea of Music with Rocks in to the Discworld.
The magic that is unleashed has more to do with lampooning record company contracts than the typical fantasy, with room for plenty of puns, an hilarious tour, and some profound thoughts on what music means to people (interpreted broadly). And also, lots of silly business with the Death of Rats and Quoth, a raven. Funny as anything, and never mean-spirited.
These books get even better with age, both mine and theirs.