I really don't know...
For fiction... OH. I KNOW NOW. "Angel Fall" by Coleman Luck. I would want to read that; it would most likely be a huge souce of comfort. Or possibly Wayne Thomas Batson's "The Door Within" books. Or his "Dark Sea Annals" books. Or... anything by him, really. All of his books would be a source of comfort in some way.
I'd also want to try and read books my friends had written too though, because (assuming I hadn't somehow managed to get them all into one place so I could spend my last day on Earth with them) I'd want to be thinking of them...
So many books.

For nonfiction... I really don't know. I don't read a lot of nonfiction. I'm going to cheat and say that N.D. Wilson's books "Notes on a Tilt-A-Whirl" and "Death By Living" count as nonfiction (technically they do; just very pretty nonfiction

), and that I'd read those.

Because they'd be comforting too.