Books
"Angel Flying Too Close To The Ground" written by Willie Nelson, performed by Jobuk Johnson.
Jobuk Johnson

Some time ago Jobuk started studying permutations, the rearrangement of a group of notes.

In four-part harmony, there are 24 different arrangements of each four notes.  Out of the chromatic scale, there are 165 four-note combinations. So if you are exploring the guitar, that will keep you busy.