Well, I learned C scale at school and never really bothered about it afterwards.
I began the guitar, playing simple chords without any question where they came from. But as I met more complex chords, and understanding some simple chords are in fact chord inversions, I went through scale study to understand chord construction instead of buying tons of books about chords... and modes.
It is true that most instructional books share the same musical roots (how couldn't they do so ?) but it is also true that some spoke clearer to me than others. I once in a while got back to some chapters I knew I did not understand well the first time. I even bought a learning aids or two, I may probably still find somewhere on my bookshelf.
What helped me much is to commit myself to explain what I learned : see one, do one, teach one ! I can now, with a pen and a simple sheet of paper, explain the scales (and mode principle), chord composition, cycles of forth and fifth applied to key (and notes) recognition on the staff and transpose it all on the guitar neck.
Yep ! It fitted in my head and probably on a (two sided !) paper. But, it did not get in after reading only one book. I had to see the thing from different points of view, as not all writers used to explain all (some would talk about diminished notes without saying we could also say "flattened".
The stuff is hard to swallow and you have to be motivated to learn it. That is why I found rewarding to learn alternating theory with exercises. And since every author brought his own exercises, my learning was even more interesting. I tried some books devoted only to exercise : sorry, not for me ! I still open some once in a while, do some exercises, close them and put them back on the shelf. As I play for my own pleasure, I prefer to work a tune : more rewarding. My learning curve was slow, but, I was not in a hurry. So, it may be different for whom who wants to make some money out of it.
P.S. I do not not know the book of Al Di Meola, but maybe I could get it... ;-)
