That takes me back - I have the books and went to a riding school in the early 90's where their entire training was based around this.
A bit focussed on using the upper leg to the exclusion of the lower leg, and teaching has moved on from there - sadly my training was ingrained and I haven't!
I wouldn't say it is outdated so much as has gone out of fashion in this country to an extent, it is still quite well supported in the US with a number of instructors still teaching to this system. In terms of general principals of balance it is still an interesting read - and it does still form the core of my riding style.
My awareness of changes in teaching is more based on what I hear from people who have learned to ride in the last 15 years or so, who appear to have more emphasis on the lower leg in particular. Although I do have a wide selection of books I haven't bought a general riding one in years (but have a whole library of dressage books