Absolutely fascinating. McDougall write for Runner's World magazine and has a very conversational, journalistic style that makes the book a breeze to read. Largely about a tribe of superrunners, the Tarahumara, and a 50 mile trail race in the Copper Canyons, the book also includes facts about the great trail runners out there, tips for distance runners, and the anthropological theory that argues that humans evolved to be distance runners.
It really is a fascinating book.