This is the 39th Discworld novel. Holy cow! It's also a Sam Vimes book, a subseries which is probably my favorite. (If you want to start reading Pratchett but don't know where to start, Guards! Guards! is the first book with Sam Vimes as a main character, and it's also an excellent starting point for the Discworld and Pratchett as a whole.)
Sam Vimes, Commander of the Ankh-Morpork City Watch, is on his first holiday, much against his will. His wife Sybil insisted on taking him and Young Sam (who is six and enraptured with a series of children's books about poo and other gross stuff) to the Ramkin family estate in the country. Vimes expects to be called back to the city at the last second by an emergency, but it never materializes. But once in the country, he doesn't have to dig very far to discover something disturbing going on--something to do with the local goblins.
The last Discworld book, Unseen Academicals, dealt with the Ankh-Morpork class system from the bottom up. Snuff deals with it top-down, with Sam Vimes--a gutter rat turned duke--typically uncomfortable having to play the country squire. It's a fascinating book with an excellent plot, but I did think it was a bit too talky. Vimes talks and thinks too much about what the class system means. It slows the action down, makes the book longer than it really needs to be, and becomes slightly repetitive.
That's not to say it's a bad book at all. Vimes is his usual pugnacious, hardheaded self, grimly determined to solve a murder and a disappearance. His relationship with Sybil is quietly touching (and funny), and Young Sam is awesome in his singleminded pursuit of new friends--no matter what they look like--and poo for his poo collection. The goblins are a great addition to the creatures of Discworld, with a fascinating religion and good reasons why they haven't cropped up in the series before now.
Unlike most of Pratchett's books, this one is told almost exclusively from Vimes's point of view. As a result, it doesn't have the huge scope of many Discworld books. I would have liked to see more directly what happens to Fred Colon, since a lot of his subplot happens off-page. But Vimes is a great character and certainly fun to spend time with.