The classic textbook of the field: Essentials of Conditioning and Strength Training.
It's full of fun facts and techniques without being overly technical. Good high school bio should suffice with a gross anatomy refresher. A little biochem won't hurt either but you get the Krebs Cycle in basic bio.
Did you know that physical conditioning includes mitochondrial growth? It does, over time. It's remarkable to learn the ways the body rises to meet stresses slowly but surely.