UltraMarathon Man: Confessions of an All-Night Runner by Dean Karnazes
I'm fairly certain that every other runner has already read this book, and that non-runners won't care, so I'll make this short and sweet.
His first attempt at the Western States 100 was a success. His first attempt at the Badwater failed. He "ran" a marathon at the South Pole. He runs 200+ miles to raise money for children in need of transplants. He runs in the morning, he runs after work, he runs through the night.
Most of all, it's just an interesting read to see what his extreme races were like, how his body felt, and the thought process through it all.