Night Watch
This is among the very best Discworld novels, along with the likes of
Small Gods.
A bit darker than normal, and with much less emphasis on the humour, this is an Ankh-Morpork Watch story.
Sam Vimes (who has grown to become perhaps my favourite Discworld character) is thrown into the past to hunt down a psychotic killer and finds he has to relive one of the formative experiences of his youth and take on the mantle of his mentor in order to heal time and get back to his present and his newborn child.
The plot is a device allowing Pratchett to really get under the skin of a character.
It’s a beautiful piece of writing.