[MisterOctober]

Former musician here --

A big +1 to the suggestion about ear training; I sure wish I'd pursued it more seriously and earlier -- probably no other activity can transform one's understanding and ability to the degree ear training can, not to mention it makes one _much_ more able to play + improvise in a group setting --

Finding a beginning book / course / etc that engagingly + logically shows the relationships between scales and chords, modes, etc, and addresses some aspects of rhythm, was immensely helpful for me -- if it uses your instrument, so much the better -- mine was Richard Chapman's "The Complete Guitarist"

Once you have the basics in hand, there are a lot of elucidating paths available to follow, but one book on counterpoint that absolutely _turned my head around_ is Joseph Fux' "Steps to Parnassus" also called "The Study of Counterpoint" -- total lightbulb --

I also strongly recommend Aaron Copland's "How to Listen to Music"