Terry Pratchett 
Men At Arms 
(Discworld Novel 15)

 »PEOPLE OUGHT TO THINK FOR THEMSELVES … THE PROBLEM IS, PEOPLE ONLY THINK FOR THEMSELVES IF YOU TELL THEM TO. »
Times are a-changing in Ankh-Morpork »s Night Watch.
New recruits have been hired to reflect the city »s diversity, including Corporal Carrot (technically a dwarf), Lance-constable Cuddy (really a dwarf), Lance-constable Detritus (a troll), and Lance-constable Angua (a woman … full moons aside).
What »s more, Captain Sam Vimes is getting married and retiring from the Watch. For good. Which is a shame, because no one knows the streets of Ankh-Morpork or its criminal underworld better than him.
And someone armed and dangerous has been getting ideas about power and destiny and lost kings, committing a string of seemingly random murders across the city.
The new recruits will need to learn fast …
 »Funny, wise and mock heroic . . . the best-crafted book I have read all year » Sunday Express
Men At Arms is the second book in the City Watch series, but you can read the Discworld novels in any order.