In my quest to become someone more knowledgeable about bicycling, and my own body, I started reading a new book today. This is a book that all of the well known coaches - most specifically, Friel and Daniels - recommend and cite.
The book is "Periodization: Theory and Methodology of Training" by Tudor Bompa, Ph.D. Dr. Bompa basically invented the concept of periodization of training around 1963, and used the theory to lead eastern European teams to world-level athletic dominance for about two decades.
The ideas are simple enough, and applies to most sports. The way you get stronger is to slightly overdo it, and let your body recover. It is through the proper timing and spacing of workouts, with tuning for your particular sport and body, that you achieve improvement.
I've just scratched the surface of this book, and will probably have all sorts of opinions as I plow through it.