This book is one of a series of karate texts written by Javier Martinez.
It fairly presents Martinez Sensei's thoughts on Nihanchi Kata (Isshinryu style) and the applications he presents. Thus the secrets revealed are from his point of view.
Isshinryu on the whole is a clannish lot of artists. What one group holds as important may not be seen that way by other groups within the system. Of course the same can be said equally about one style versus another style too. Thus it is difficult to answer any authors worth, it depends on the reader's point of view.
In this and all cases, Martinez Senei and others must be commended. They are publically expressing their knowledge. That is a challenging issue in all days and it will always give one pause to reflect on their answers. The true worth of any book.
Thus the relevence of Martinez Sensei's answers depends on one's point of view.
I believe all of his works were private printings and have heard no discussion about further re-issuing of this work.
Another possiblity is Motobu Choki's "Karate My Art" translated by Patrick McCarthy. Motobu Sensei was one of the instructors of Isshinryu's founder, Shimabuku Tatsuo.
bushi no te isshinryu