When it arrived I was expecting another book of scales and licks. But was at first disappointed at the lack of, um, scales and licks. This book was not like any other guitar book I'd ever seen. It wasn't another glossary of lines to develop your left and right hand chops. It was a book written to open your mind and thus your hands with lessons designed to get you out of the boxes we tend to climb into as we learn.
How often do you pick up your guitar, play a song or a solo, and think "I hate my playing" or "I wish I had some new ideas"? I know I have. Normal books would give you a bunch of licks (ideas) to memorize. But The Advancing Guitarist gives you new ways to think about how you play the guitar. It takes a minute for the light to turn on sometimes. But often it it's easy to see application in the concepts.
A lot of the book is dedicated to harmony and voicings. Probably the main takeaway for me.
On a related note check out my video on combinatorics...