Mort is the fourth Discworld novel, and it's about Mort, a young man who finds himself apprenticed to Death. Mort is a bit clumsy and too much of a thinker for his farmer father, but earnest. Unfortunately, on his first run harvesting souls on his own, he's sent to collect a beautiful princess - but instead of doing so, he saves her from the assassination attempt - and thus alters reality on the Disc, sending repercussions throughout the Disc.
Pratchett's personification of Death is one of his best and most enduring literary creations, and Pratchett has a good time here with Death being tired of his job (which has, quite literally, lasted for eternity). So he gets to turn many of our ideas about death (and "Death") on their head, to fairly humorous effect.