Pratchett's Discworld books can be further subdivided into several series starring various lead characters (Rincewind, Granny Weatherwax, Death, etc) - Guards! Guards! is the first of the 'Watch' books, which taken en masse are probably the best Discworld books of all. The story concerns a plan to depose the current ruler of Ankh-Morpork by using magic to summon a dragon to terrorize the city, and stage manage a false hero to defeat the menace and be crowned king - needless to say these plans go wrong, and the summoned dragon cannot be controlled, and takes Ankh-Morpork for itself...
The novel introduces the recurring unlikely heroes of the Night Watch, a ragged bunch of city guards who, thanks to the official policing of thievery by the Thieves Guild, aren't actually supposed to do anything normally. Captain Sam Vimes is ostensibly the lead hero character, though it's interesting looking back at his first novel that the character is more of a jaded alcoholic than the 'super-policeman trapped in a diplomats body' that the character would develop into. Other new faces that would go on to feature in numerous novels include the cynical Sergeant Colon, the loathsome Corporal Nobbs, and new recruit Constable Carrot - a human raised by dwarves with a heroic background only hinted at here. Other notable first appearances include the Patrician Lord Vetanari - while the character had been named in previous books this was the first time the character actually took on a central role, and became the familiar all knowing, all-scheming ruler of Ankh-Morpork, and street vendor Cut-Me-Own-Throat Dibbler, though the character was initially more of an all-round spiv than just a seller of dubious pork products.
Guards! Guards! has a plot that rattles along at a good speed, plenty of memorable characters, and is one of Pratchett's funniest novels (the 'It's a million to one chance - but it might just work' sequence being possibly the best extended gag). For those who've read the previous 7 Discworld novels this is the best one yet, and thanks to its nature as the first 'Watch' book this is also a great jumping on point for new readers. Highly recommended.