Previously my running bible was CHI Running, but I just finished reading
BORN TO RUN: A Hidden Tribe, Superathletes, and the Greatest Race the World Has Never Seen (Christopher McDougall). Not only was it a great read and great story, it is truly inspiring on the runners.
According to evolutionary biology we humans were built to RUN! The long and the short of the book is that Running made us smart, smartness made us runners and so on, plus a bunch of nice lovey dovey stuff about how running is love and love is running.
HIGHLY RECOMMEND the book!