After a decade of research, Theodor Hettinger published Physiology of Strength in 1961. This is a great book if you want to learn the ins and outs of improving your fitness level. This classic is well researched with tons of published articles that support his findings.
At the end of the day, all Hettinger really wanted is to improve the physical capacity of everyone who read his book so that they can live their life optimally!