Soul Music by Terry Pratchett is decidedly weaker then the other books in the Death sub-series of Discworld books. Like the other Death books, Death has taken a break and something must be done in the meantime. This time, however, it results in a disjointed narrative that isn't quite up to Pratchett's standards.
Death is feeling extremely melancholy. He wants to know what it's all about, and what humans do to forget. After getting some not-so-helpful advice from his manservant, Albert, he decides to go on a journey to forget everything. Thus, he ends up in the Klatchian Foreign Legion, a place so forgetful that the soldiers forget what they're saying in the middle of saying it. Somebody must take over his job, however, and this time the task falls to his granddaughter, Susan. Susan has been made to forget everything about being Death's granddaughter, so when Death's assistant, the Death of Rats, comes to recruit her, she's very skeptical about the whole thing. Gradually, though, she starts to remember things and decides that she can do the job. But she starts suffering attacks of conscience, not wanting to follow the job description. She wonders why good people must die when they do, and why bad people can't die sooner, etc.
She stumbles upon Imp y Celen, a bard who is supposed to die but is saved by some mysterious force. Imp, along with a dwarf and a troll, have seemingly brought a new form of music to the Disc. It's Music with Rocks In. It's got a great beat and you can dance to it. The crowd goes wild, throwing various undergarments onto the stage. The problem is, though, this music is alive. And it wants to stay that way -- no matter what it does to a mere bard. Or civilization, for that matter.
There's an upside and a downside to Soul Music. It is tremendously funny, with wonderful homages to various rock and roll elements. There are numerous Blues Brothers riffs, lots of Meat Loaf references, and various parodied song lyrics and band names. "'A song about Great Fiery Balls. [...] Couldn't really make out the words, the reason bein', the piano exploded.'" These sorts of references permeate the book and make it a joy to read. Some of the band names in the book include "We're Certainly Dwarfs (They Might be Giants), "Suck" (Kiss), "The Surreptitious Fabric" (The Velvet Underground). I'm sure you can see where this is going.
On the other hand, the Death aspect of this story covers ground that's already been covered, to an extent. Yet again, Death has decided to walk away from it all and visit humanity. Once again, somebody has to take over his job. Yet again, the person who does so has trouble with the "moral" aspect of it. In Mort, the title character fell in love and couldn't bring himself to "kill" a little girl. In this one, Susan has qualms about "killing" innocents and letting bad people live out a full life. Why can't it be the other way around? It feels very recycled, and it's only saved in Soul Music because Susan is interrupted by the onset of this strange music that's making everybody go weird.
The other problem with the book is the incredibly disjointed feeling that it has. Reaper Man had the same problem, but that book was so good that it made up for it. This time it's even worse. The Death storyline doesn't get much time, with only a couple of scenes with the Foreign Legion, then a couple of scenes back in Ankh-Morpork before finally being called back. Susan's story takes forever to begin, as she is first approached by the Death of Rats, is skeptical, has it shown to her, explores Death's house, etc. It gets to the point at the beginning of the book that it comes to a screeching halt whenever the story leaves the music and goes back to Susan.
All of that being said, though, Soul Music is still a very entertaining book. The characters are wonderful; only Susan is slightly boring. The other characters are either very well-developed or just in there for the sake of the joke (like the band that continually changes its name because, well, they stink and they can't get any recognition). The three main band members (Imp, Glod and Cliff) are great, all true to their racial characteristics (dwarves love gold, and so does Glod, for example) yet with wonderful personalities. The wizards are back yet again, wreaking havoc with everybody and everything. All but Archchancellor Ridcully seem to be taken over by this new type of music, and the scenes between Ridcully and the Dean are priceless (the Dean acts like a child who is being denied while Ridcully is the father-figure sending the Dean to his room without any supper). The jokes come fast and furious whenever the wizards are around. Death, when he is around, is as funny as always. There's something about a personification of an aspect of life "trying to forget" that's really funny, especially as executed by Pratchett.
Next to Reaper Man, Soul Music comes up a bit short. It's still entertaining, and Pratchett fans will still enjoy it. Read a few Discworld books in between the two, though. If you don't, you may find yourself feeling worse about Soul Music then you actually should.