I have a read a significant number of books about fitness. They range in quality and in genre. Some are essentially reprints of complicated protocols used by coaches. Others attempt to give training advice based upon evidence, whether scientific, anecdotal, or testimonial. Some attempt to give theories of training from principles. The two books that have been most useful to me in that regard are below (A close third is Starting Strength Mark Rippetoe).
McGuff, Doug, and John R. Little. Body by Science: A Research Based Program to Get the Results You Want in 12 Minutes a Week. New York: McGraw-Hill, 2009.
Many people might think this book is bogus, especially because of the subtitle. But the book is a marvel of attempting to use scientific research that really is not about training protocols in order to come to conclusions about the human body. Those conclusions are used to infer the most efficient training program available. Thus the system is brief, simply, hard, and safe. This is also one of the best books for pointing out the rather comprehensive benefits of strength training.
When the subtitle says, “12 minutes a week” it does not mean only twelve minutes in the gym. It means doing each set of exercise to muscular failure for one hard set after a warm up. Thus your squats performed in this fashion might be a set that takes one minute of smooth reps taking 4 seconds each for 12-14 reps. After training in this fashion you will need to rest briefly before your next exercise. But with adequate rest (less than the book prescribes…I’d say to train this way between 1 and 4 times a week) you will make progress. So the twelve minutes is referring to the fact that a six movement routine will, ideally require 6 minutes of time actually lifting the weight after a warm up. Doing such a routine twice a week is 12 minutes of training.
If you wish to improve your overall strength and fitness in a short time I recommend reading this book and using Johnny Candito’s youtube channel to learn proper form.
Perryman, Matt. Squat Every Day Myosynthesis, 2013 Kindle Book
Perryman’s book is superb. He challenges several misconceptions by trying to look at the nature of the human person in a “meta” sort of way. You are a dynamic system so training your body in increments makes sense, but it does not necessarily work like this “Stimulus, rest, adapt, repeat.” He recommends that to make long term gains it is useful to expose your body to difficult but not impossibly difficult stimulus on a regular basis, like every day. This sort of protocol will not be best for saving time, but it might be best for injury recovery (due to a weird feature of connective tissue), psychology (you don’t have to psyche yourself up to lift heavy weight if you do it every day), and strength (because the movements are trained so often that they actually become a skill).
These books, though widely divergent in conclusions, might be the best books that are easy to read for understanding the human response to exercise. Both approaches work and both are based upon the same principles. Depending upon your goals and values (time/strength/soreness/nagging injury healing, etc) you can use one or a combination of both approaches to approach your desired level of fitness.