Mort, by Terry Pratchett.
Okay, I've been loving the Discworld novels, but I really feel that Pratchett is starting to hit his stride with this one.
What happens when Death seeks an apprentice? It may be that only Pratchett would think to ask, and even more likely that only he would come up with an answer that makes me and my 13 year old both laugh aloud (though not necessarily at the same bits. It may be best not to know sometimes). If The Colour of Magic reminded me a lot of Douglas Adams, with Rincewind about as much in control of his destiny as Arthur Dent, Mort is more intentional and funnier, even while it has a darkish theme (though less dark than you might expect with Death as a main character).
The lead character, Mort, becomes Death's apprentice by chance and default, but he is not content just to be blown by the winds of fate. He struggles from the beginning to take control of his destiny, and if he screws it up big time (nearly destroying the world, or possibly the universe, in the process), he does it with a reasonable idea of the outcome he wants. And when fate seems to be overcoming his desires, he pulls out an impressive amount of will-power to remain his own man.
Mort's love-life is perhaps the least convincing aspect of the story, yet even that feels to me very real--he's a teen, with limited ideas about what love should look like. I won't say more on that subject lest I offer a spoiler. (Note to self: ask teen sons what they thought of the romance. If they noticed it.)
I gave the book four stars because I save five for perfection, and I think Pratchett will get there. But I'd make it 4.5 if I could.
Recommended for readers who like humor, from about age 12 up.