From the Chapter I, Introduction:
"The following pages are my interpretation of my acquired bit of stretching knowledge and experimentation. It is my hope to show you how to combine the knowledge gained in the works such as Relax Into Stretch and Super Joints as well as some homegrown stretching advice gleaned over the course of my journey. It goes without saying that if you have not read the workes mentioned above, STOP and order the immediately. They will be reference heavily here and you simply cannot be without them."
We need to take "bit of stretching knowledge and experimentation" with a grain of salt here - it's a lot more than a bit. Jon's journey represents a lifetime's voyage that, to date, has led to a 7th degree black belt in TKD and high ranks in several other disciplines as well.
I think of Flexible Steel as what Pavel might have written next if he decided to boil down RIS, SJ, and others into a simple, practical approach to joint health, mobility, and flexibility, much as he has done with Kettlebell: Simple & Sinister.