I first heard about RAMDK from a pastor a few years ago. He was making up for lost time with his son after putting in long hours finishing seminary and starting a church. It's easy for pastors to get sidetracked by the Church at the expense of their families.
The most well-known book I know of right now on raising a son from a Christian perspective is Dobson's "Bringing Up Boys." While that is an excellent resource with a lot of valuable research, it lacks concrete, creative ways to shape one's child. It sits half-read on my nightstand. Lewis' book is also published by Dobson's Focus on the Family-- who seem to have the most positive impact on culture when they stick to Focusing on the family.
Robert Lewis defines a man as "someone who reject passivity, accepts responsibility, leads courageously, and expects a greater reward." (p.61) I think that is a pretty solid definition, and Lewis uses examples from Scripture to support his many concepts. If there is one criticism I have, it's that Lewis indeed has MANY concepts: 3 Shortcomings...10 ideals...4 principles, etc. This guy loves lists and bullets.
An additional concept that some might challenge in Lewis' plan for modern-day knights, is his focus on loving a woman. In my own life, loving my wife has defined much of who I am as a man, but I'm not sure that every man will find the love of a woman as a key component of his service to Christ. The Apostle Paul is a good example.
Key things that I thought were helpful were Lewis' promotion of the "transcendent cause" as a source of meaning in men's lives, his story of "working for a King," and his support of being a part of "the community of men" for the sake of ourselves and our sons.
Lewis' basic premise is that raising a modern day knight requires instilling the following in our sons: 1) A Vision of Manhood 2) A Code of Conduct 3) A Transcendent Cause. I think this book will prove to be helpful for fathers as well as their sons, as many from my generation have grown up with "destination sickness--" Lewis' term for the feeling one gets when he's arrived at his goal only to find it lacking.