Night Watch by Terry Pratchett (Discworld series #29)
Sam Vimes, commander of Ankh-Morpork's City Watch, is chasing a killer named Carcer during an intense storm. During the pursuit, both Vimes and Carcer become the victims of a magical accident that transports them back in time 40 years. If Vimes wants to return to his rightful time and rejoin his wife who's expecting their first child, he must make sure things happen the way they originally did 40 years ago.
That becomes very unlikely when Carcer murders John Keel, the Sergeant responsible for training Vimes to be a good copper 40 years ago. Now Vimes has to take on the identity of Keel and train his younger self, uh himself.
Night Watch is Pratchett's 28th story which takes place on Discworld, a flat world that rests on the shoulders of four elephants which in turn are standing on the shell of A'Tuin, a giant turtle swimming through space.
The books have ranged from good to fantastic and Night Watch is one of the best so far. I thoroughly enjoyed the chance Pratchett gives with this one to see what many of his characters were like 40 years ago.
As always, Pratchett's humor is there, but it's no longer the primary reason I read his books as it once was. With each successive book, it's more apparent just how brilliant he is. On the surface his books are lighthearted fantasy novels. But deeper down, they're amazingly insightful social commentaries. They're the types of books that should be read by everyone.