Zen guitar achieves what every other Guitar tuition book fails to do, and that is to provide the guitarist with a sense of 'soul' or 'depth' to their playing. It is certainly spiritual, and at times, eclectic, but the concepts and thoughts of the late, great Philip Sudo have inspired many guitarists into approaching the guitar in several different ways.
A prime example of how a simple instruction yields incredible results. In the book, the instruction is just to "Pick up the guitar, and play each note as if it is the last one that you'll ever play". Coupled with a few sage paragraphs that get you mentally prepared for this task, and with a little help from many great quotes from many great guitarists dotted around the book, suddenly, the expression and passion that you play the instrument becomes tenfold.
There is not one piece of notation, or explanation of music theory. The principles throuought "Zen Guitar" are profound, and may alter your outlook beyond the fretboard, and every word has been written with the reader's best interests in mind. No matter what you are trying to achieve with the guitar, no matter what level of proficiency you believe you have achieved, and no matter how many books or how much time you have studied to get there, this will be a book that you need to have in your collection.