For the past 10 years, Craig Weller and Jonathan Pope have trained people for the toughest SOF selection courses in the world. They've trained candidates for courses in every branch of the U.S. military, as well as five other countries, including British SAS selection. Their clients have a 90% success rate which means 9 of 10 people make it through. (BUD/S and SFAS have drop-out rates ranging from 60-90%, FYI.) In Building the Elite , they give a detailed look at their SOF training system - from big-picture principles to practical exercises and programming guidelines. This is a 538-page textbook, with in-depth chapters on a variety of topics that you'll reference for years. Topics covered systems theory, complexity, movement & physical resilience, psychological performance, stress inoculation training, managing pain & fatigue, skill acquisition, mental skills, nutrition, strength training, running, rucking, swimming, and building workouts.
For a narrow niche but honestly I'm surprised at the sheer volume and depth of information provided. You'd pay thousands of dollars to get the stuff given in the book and their website. They are the unquestioned experts in this topic and the knowledge and experience shared is unrivaled.