Reaper Man: Book 2 in the Death arc.
Death - Exploring the afterlife with DEATH himself... and his skeletal horse Binky... and his daughter. If mass extinction, end of world scenario’s and anthropomorphic personifications tackling human emotions sounds like your thing look no further.
'Death has to happen. That’s what bein' alive is all about. You're alive, and then you're dead. It can't just stop happening.'
But it can. And it has. Death is missing – presumed…er…gone (and on a little farm far, far away, a tall dark stranger is turning out to be really good with a scythe). Which leads to the kind of chaos you always get when an important public service is withdrawn. If Death doesn't come for you, then what are you supposed to do in the meantime? You can't have the undead wandering about like lost souls. There's no telling what might happen, particularly when they discover that life really is only for the living...
The Auditors of Reality are beings who watch the Discworld to ensure everything obeys The Rules. As Death starts developing a personality the Auditors feel that he does not perform his Duty in the right way. They send him to live like everyone else. Assuming the name "Bill Door", he works as a farm hand for the elderly Miss Flitworth.
While every other species creates a new Death for themselves, humans need more time for their Death to be completed. As a result, the life force of dead humans starts to build up; this results in poltergeist activity, ghosts, and other paranormal phenomena. Most notable is the return of the recently deceased wizard Windle Poons, who was really looking forward to reincarnation. After several misadventures, including being accosted by his oldest friends, he finds himself attending the Fresh Start club, an undead-rights group led by Reg Shoe. The Fresh Start club and the wizards of Unseen University discover that the city of Ankh-Morpork is being invaded by a parasitic lifeform that feeds on cities and hatches from eggs that resemble snow globes. Tracking its middle form, shopping carts, the Fresh Start club and the wizards invade and destroy the third form, a shopping mall.
When humankind finally thinks of a New Death, one with a crown and without any humanity or human face, it goes to take Bill Door. Death/Door, having planned for this moment for some time, outwits and destroys it. Having defeated the New Death, Death absorbs the other Deaths back into him, with the exception of the Death of Rats (and ultimately, the Death of Fleas). Death confronts Azrael, the Death of the Universe, and states that the Deaths have to care or they do not exist and there is nothing but Oblivion, which must also end some time.
Death asks for and receives some time. He meets up with Miss Flitworth again and offers her unlimited dreams. She asks to go to the local Harvest Dance. They prepare and join the townspeople for a full night of dancing.
As the sun is coming up, Miss Flitworth realizes she had died hours before the dance even started. Death escorts her through history to her old fiancé. Returning to the city of Ankh Morpork he meets up with Windle Poons, finally taking him to his just reward, whatever it is. At the end there is also a discussion between Death and the Death of Rats over what the Death of Rats should "ride", Death suggests a dog while the Death of Rats suggests a cat.