Mort: Book 1 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 comes to us all. When he came to Mort, he offered him a job.
Henceforth, Death is no longer going to be the end, merely the means to an end. It’s an offer Mort can't refuse. As Death's apprentice he'll have free board, use of the company horse - and being dead isn't compulsory. It's a dream job - until he discovers that it can be a killer on his love life...
As a teenager, Mort has a personality and temperament that makes him unsuited to the family farming business. Mort's father Lezek takes him to a local hiring fair in the hope that Mort will land an apprenticeship; not only would this provide a job for his son, but it would also make his son's propensity for thinking into someone else's problem.
At the job fair, Mort at first has no luck attracting the interest of an employer, but just before the stroke of midnight, a man wearing a black cloak arrives on a white horse, saying that he is looking for a young man to assist him in his work and selects Mort for the job. The man turns out to be Death, and Mort is given an apprenticeship in ushering souls into the next world (though his father thinks he's been apprenticed to an undertaker).
When it is a princess' time to die (according to a preconceived reality), Mort, instead of ushering her soul, saves her from death, dramatically altering a part of the Discworld's reality. However, the princess, for whom Mort has a developing infatuation, does not have long to live, and he must try to save her, once again, since the original reality will eventually reassert itself, killing her in the process. Both the princess and Mort end up consulting the local wizard, Igneous Cutwell, for various methods of assistance with the crisis.
As Mort begins to do most of Death's "Duty", he loses some of his former character traits, and essentially starts to become more like Death himself. Death, in turn, yearns to relish what being human is truly like and travels to Ankh-Morpork to indulge in new experiences and attempt to feel real human emotion with Happiness being the one he finds hardest to understand and so starts some research to try out happiness, something that he has never experienced, he tries a number of very human habits like getting drunk, going to a party, dancing and finding a job.
Mort, as well as Death's adopted daughter Ysabell, discover that Albert, Death's manservant, is in fact Alberto Malich, a centuries old wizard who has been living with Death in order to put off his own demise, due to the fear of enemies awaiting him in the afterlife. With reality in danger due to Death's absence, Albert returns to Unseen University and has the wizards perform the Rite of Ashk-Ente, which summons both the part of Death that has been taking Mort over, as well as Death himself. Death becomes furious when he learns about Mort's actions, including seducing Ysabell, and fires him. Conclusively, Mort must duel Death for his freedom. Though Death wins the duel, he spares Mort's life and sends him back to the Disc.
The princess is saved from a second death when the alternate reality Mort created is reduced to a pearl-like state, after Death "has a word with the gods". This pearl is given to Mort for safe-keeping. At the end of the novel, Mort marries Ysabell, having been elevated to the title Duke of Sto Helit by Queen Kelirihenna.