The Healthy Dead by Steven Erikson

A pair of irreverent necromancers and their put-upon manservant arrive in a plague-ridden city where a pious movement promises miraculous cures that mask a foul, unnatural truth; driven by curiosity and cynicism, the necromancers probe rituals, conduct grim experiments and unravel lies and undead horrors, producing a darkly comic, satirical tale of magic, corruption and unsettling consequences.