I was recommended this book by a friend who was working under the authors tuition (he and numerous others have since got into high-ranking conservatories thanks to his teachings, as detailed in this book). As stated, this is a 'method', providing suitable materials, and demonstrating what is possible if they are fully absorbed through the suggested practice. If taken seriously, you will never need another book on improvisation; it is all here. Chapters deal with chords, scales, harmony and substitution, melodic development, melody, form, rhythm, and how to make the most of practice time. It is not instrumentally specific - I am a guitar player, and have improved more in the last 6 months than I have in the last 9 years (and I have been a guitar tutor for 4 of them) thanks to 'The Method'.