Spares by Michael Marshall Smith is a book where the title has little to do with the main plot of the book and it's clear that the author didn't quite know what he was doing as he wrote it.
The initial premise of the book is that when embryos are first formed in human mothers and the process of mitosis begins, a number of cells are harvested and grown artificially into a person with an identical genetic make-up to the person who the mother births naturally. This carbon-copy person is kept in a series of tunnels with no education or interaction with the outside world, except for on those rare cases when the original needs a spare body part. At that time, the carbon-copy's body is harvested for the needed limbs or organs. These carbon-copy bodies are known as spares. That's where the book starts. It then shifts track radically to become your standard crime who-dunnit with a disgraced cop fighting his demons (his wife and child were murdered, of course) and attempting to right past and present wrongs. Along the way, his former partner is killed, so he gets diverted again, to solving the case his partner was working on (even though the protagonist still isn't a cop). Along the way a number of characters typical of this style of book are introduced, as are numerous ho-hum, seen-it-before situations, including the ever popular "Protagonist Must Partner With Villain Who Probably Killed His Family And Former Partner To Right Greater Wrongs". Did I mention that his buddies include the mid-level gangster who also runs a bar and the hooker with the heart of gold (who just happens to fall for him)?
Then, the book jumps the track again, and the protagonist is back fighting in a crazy world between worlds in a Vietnam-style engagement. Does this have any connection to the previous two-thirds of the book? Uhh, not really. That obviously didn't stop Smith and he roars ahead into this goofy segment with abandon.
Where are the spares, the title characters, during all this? Mostly dead. They've pretty much been dead or ignored for all of the previous two segments.
In the end, Smith uses a hasty and silly ending in an attempt to gather all of this mess up into a bowl and make some sort of literary goulash where the sum of the whole will be greater than the individual parts. Unfortunately for the readers, he fails miserably.
Unless you're trapped on an airport tarmac due to the corporate malfeasance of an airline, skip Spares. There are many better books out there.
On a tangential note, who are all the people who rated this book so highly on Amazon.com? Five stars?!? I'm not aware of a substance, legal or otherwise, that could mask the literary wrongs of this book.