Can you tell us about this or post a link? I'm not familiar with who Käppel is.
Certainly. Hubert Käppel, a German classical guitarist (Professor at the Conservatory in Köln) who wrote a book called "The Technique of the Modern Concert Guitar." The English translator mistranslated the title "The Bible of Modern Guitar Technique." This book is rapidly becoming the standard of modern guitar technique. There is nothing revolutionary here. Mr. Käppel's technique is evolutionary, representing the current state of modern classical guitar technique. The reason this book is having such a spectacular impact is because Mr. Käppel goes into great detail to explain what the techniques are for, the exact mechanics that are required, and very detailed microexercises that make mastering techniques easier by breaking them down into their component parts. I have had the book only two months, and it has already greatly improved my awareness and playing ability. For the first time in 50 years I have an idea what I am supposed to achieve by shaping and polishing fingernails (namely, first of all, quick release). There are a hundred other things that I know understand for the first time, too many to mention. One more example: you have to watch the distance between your palm and the bottom edge of the fingerboard. Mr. Käppel gives exact measurements that lead to the greatest power in the fingers with the least effort. Another example: he explains exactly how the thumb should move according to what string you are playing. The book gives hundreds of such insights, all of which contribute to better technique, that allows you to do more and do it faster.