Mort, by Terry Pratchett

Terry Pratchett's character Death, to borrow the words of his new apprentice, is "tall, wears black, he's a bit ... skinny...."

The new apprentice is a young man named Mort. ("What a coincidence," Death says when they meet.) No one else at the hiring fair was interested, so with some trepidation, Mort agrees to apprentice himself to a skeleton who shepherds souls after they die.

The job comes with perks, like riding Death's noble steed Binky, but it also comes with difficult responsibilities, like standing aside while an assassin kills a princess. When Mort chooses to interfere with that assassination, he accidentally opens an alternate dimension.

The sprawling Discworld series has many good entry points, and this is one of my favorites, because it's the first book where Death is a main character.