Terry Pratchett 
Guards! Guards! 
(Discworld Novel 8)

»NOBLE DRAGONS DON»T HAVE FRIENDS. THE NEAREST THEY CAN GET TO THE IDEA IS AN ENEMY WHO IS STILL ALIVE.»
The city of Ankh-Morpork is in turmoil, its citizens revolting. Again.
A shadowy secret brotherhood has summoned a dragon to spread terror throughout the city, intent on overthrowing the Patrician and ruling in his place. Too bad the dragon has ideas of its own …
It»s up to Captain Sam Vimes and the ramshackle Night Watch to stop it. Only problem is, the Watch are more used to dealing with mobs than dragons.
And if they can»t bring down this fire-breathing tyrant and reinstate their own, slightly less dangerous one, Ankh-Morpork might be lost. For ever…
»This is one of Pratchett»s best books. Hilarious and highly recommended» The Time
Guards! Guards! is the first book in the City Watch series, but you can read the Discworld novels in any order.