Terry Pratchett 
Snuff 
(Discworld Novel 39)

”THE JURISDICTION OF A GOOD MAN EXTENDS TO THE END OF THE WORLD.”
Commander Sam Vimes of the Ankh-Morpork City Watch is having some time off. Apparently.
But crime doesn”t take a break – it”s a truth universally acknowledged that a policeman on holiday would barely have time to open his suitcase before he finds his first corpse.
In the seemingly peaceful countryside, Vimes discovers much more than a body in the wardrobe. For the local nobles are hiding a deep, dark secret. There are many, many bodies – and an ancient atrocity more terrible than murder.
Vimes is out of his jurisdiction, out of his depth and out of his mind. But never out of ideas. Where there is a crime there must be a punishment.
They say that in the end all sins are forgiven. This might be the exception …
”As effortlessly, generously funny as only Pratchett can be, Snuff doesn”t stint on laying bare the darker side of life either” Sunday Times
Snuff is the eighth book in the City Watch series, but you can read the Discworld novels in any order.