As I've mentioned in previous guitar book reviews, the value of lesson material must be kept in mind with regards to your level of understanding, playing ability and in relation to what your ear is both comfortable and capable of hearing and comprehending. That being said, most books will have some value but choosing one that has the most value for where you are as a guitar player is key (or you can have an entire library of lesson books that you never start or finish). I am a big fan of stuff that Don Mock has done in Guitar Secrets. I got this one and the Melodic Minor as my private instructor has been pushing me to develop these abilities in my improvisational efforts over jazz standards, chord changes. So both these books (harmonic and melodic minors) have been very helpful to discern where and how to use these scales. Specifically to the Harmonic Minor Revealed -- there's a solid explanation to the harmonization of the minor scales that put a great deal in perspective, there's a listing of the 5 harmonic minor scales (but there is an error, the third pattern is a repeat of the second patter in my copy-- so there's only 4), a list of the all the arpeggios for the harmonic minor chords, some great examples and some "out there" kind of stuff also. Like all decent books, there's a level of digestion and incubation that has to occur and you have to be careful to what level you get involved with details that may not necessarily advance your playing in an efficient fashion (again, depending on where you are as a player). The challenge would be to use the harmonic minor scale fluently in an improvisational approach. I often wish these books had a few more "in between" exercises. That is some approaches that bridge the gap from understanding what their taking about ("getting it", at least theoretically) and being able to apply it effectively ("really getting it", as a player). All that said, the Don Mock series is part of my routine learning approach.