Equal Rites
by Terry Pratchett (Victor Gollancz, 1987); audiobook read by Celia Imrie (Isis, 1995)
Not the most substantial of storylines, yet a key work in the development of the Discworld. Pratchett sets Rincewind aside in favour of the far richer character Granny Weatherwax. In so doing he makes societal change a serious part of his worldbuilding.