I have been a big fan of Mark Sisson and his very sensible, well-reasoned, and comprehensive approach to living our lives so that they more closely align with the way our bodies were meant to be -- good food, adequate sleep, ongoing moderate exercise with occasional intense spikes, and intermittent fasting . His basic book (Primal Blueprint) is very easy to read and his website (Mark's Daily Apple)continuously provides information on a very wide range of topics. What I like about Mark's approach is that it seems to be based on good science (he has a degree in biology) and really isn't extreme -- his approach to lifestyle is that we make a number of mostly small changes most of the time.
There really is not much new in the book if you have read "The Primal Blueprint" or delved into Mark's website, but it is organized in very easy to follow sections that tell you about various things you need to do to become primal and then concludes with the 21-day plan, i.e., the 3-4 specific things you should do each day over 3 weeks. This really is not a one-time, three-week program though but the roadmap to transition to some long term lifestyle changes that bring our bodies and routines back to where we have been for most of our specie's existence (with room for the occasional dessert!).
I would highly recommend this book if you want a clear guide to how to shift to a primal lifestyle (you will feel better for the effort!) or want to help someone else easily adopt the primal lifestyle. It is easy to read and quite comprehensive, though you can always go to the "Primal Blueprint" or better yet I think Mark's website for more information.
One of the rare books I would give 6 stars to, though mostly for the excellent content!
PS I personally have done well at age 50+ by going primal and losing (and keeping off) some weight, getting stronger with fewer trips to the gym, and developing a lipid profile which my internist says is one of the best he has ever seen (good cholesterol consistently close to 100) and going from possible pre-hypertension to a consistent 110/70 blood pressure reading. All the while enjoying great food like buffalo burgers, loads of vegetables, eggs, nuts, good chocolate, etc.