I bought Darrel Gabels instructionals on
legato technique. They were very useful
in getting a grasp on advanced
Allan Holdsworth style playing. I learned a lot about unusual chords as well as scales i'd never think of using-whole tone,augmented, the real diminished scale(h/w or w/h formula). Anything that makes you see music in a new or different way can only be good in the long run.
Consider it money well spent.
All the Best,
Tony