Interesting. This book isn't listed on Amazon - not even as "out of print." I have no idea why.
Susan Sto-Helit is the daughter of Mort, Death's apprentice, and Yvelle, Death's adopted daughter. Apparently, there's more to heredity than DNA, because when Death gets depressed and takes a break, Susan gets the job. Which she thinks is extremely silly, since her parents brought her up to be sensible. It puts her in a good position, however, to witness the breakout of Music With Rocks In, a wild music set free by an enchanted guitar.
The book is full of underhanded cultural references, such as a joke that makes reference to Thelonius Monk, or the dwarf band "We're Certainly Dwarves." While these are not necessary to appreciate the book, they add a lot. Of course, there's certainly enough hijinks to keep you otherwise amused.