The best book I have ever read in regard to the physiology of training for endurance sports is called Better Training for Distance Runners by Peter Coe (Seb Coes Dad). It does require a scientific background to understand the book completely, but it is well worth the read. I would not however follow Seb Coes training plan which is detailed in the book. It would melt 99% percent of athletes into the ground...

***Sorry, should also give credit to the other author, David Martin PhD, a physiologist.