Wyrd Sisters (first Witches novel in which the characters have more or less settled down to what they’ll be for the rest of the series) or Guards, Guards (first

Wyrd Sisters (first Witches novel in which the characters have more or less settled down to what they’ll be for the rest of the series) or Guards, Guards (first Vimes novel, and first ‘normal’ Ankh Morpork novel; an Ankh Morpork shows up in earlier novels but that version is more or less entirely retconned away quite quickly.)