The premise of Discworld is spread across all of Terry Pratchett's books, but there are encapsulated domains within, albeit with nodding reference to the others, which makes groups of stories unique in character featuring the Witches, Rincewind, Death (the character) and these, the City Watch. The differences in character between these collections manages to keep the writings fresh and interesting, and rather than thinking that you have to read all 37, 38 (?) books to "complete the set", you are drawn to certain scenarios. Collected together in these trilogies is a wonderful way to revisit and rediscover Pratchett's genius.