Soul Music is (along with Hogfather) my favourite Discworld novel. I first read it at the age of twelve, and finally, five years later, someone got it through their heads to get it for me for Christmas.
The book revolves around a young man called Imp y Celyn (who's name translates roughly to "bud of the holly"), who journeys to Anhk-Morpork in a bid to become the greatest musician in the world. In the city, he meets a troll named Lias (who is incapable of counting to four) and a dwarf named Glod Glodsson (who's only in it for the money), and together, they form The Band With Rocks In. Thier style of music, dubbed "Music With Rocks In", takes the Discworld by storm, causing it's inhabitants to become obsessed with songs such as "Don't Step On My New Blue Boots" and "Good Gracious Miss Polly". Even the wizards in the Unseen University have been tranformed by it, with the Dean painting his bedroom black and weaing a studded leather robe that says "BORN TO RUNE".
Many aspiring Music With Rocks In bands spring up in The Band With Rocks In's wake, such as "We're Certainly Dwarfs" and a band that changes it's name so much they just end up being known as "Ande Supporting Bandes".
Cut-Me-Own-Throat Dibbler also appears, eager to exploit the new fad to make himself a quick fortune.
Only thing is, Music With Rocks In is alive. And it's the only thing stopping Imp from meeting an early death.
Meanwhile, Death has joined the Klatchian Foriegn Legion in a bid to forget, and it's fallen to his granddaughter, Susan Sto-Helit, to take on the Duty. And she's more interested in saving Imp from his "Live fast, die young" destiny, which causes a lot of trouble for Albert and the Death of Rats.
All in all, Soul Music is a hilarious book and one of the best in the Discworld series, with many puns on well known aspects of music.