the third Watch novel. Next one after Men At Arms. And much less of a gap in Discworld than the first two -- he apparently found a lot more to stay. Spoilers ahead for the earlier ones. (Also there's a scene that makes sense only after Reaper Man.)
After the increase at the end of the last work, the Watch is bustling. Vimes hires an alchemist to do forensics. Two old men are found, murdered -- one a priest, one who kept a museum of dwarf bread. And it looks like a golem is responsible.
Then it appears that Vetinari is being poisoned. Which is very much the Watch's duty to put a stop to before he dies.
This involves a dwarf wearing a silvered chainmail vest, a vampire Dragon King of Arms, locating arsenic in a surprising place, Carrot's drawing his sword just to make a point, discussing the possibilities of succession, rat-hunting in AnkMorkph, and much more.