"Let me cound the ways" - kind of an accumulation of things -
After high school, the Army (don't laugh - you never know who you will meet that can help - or say something that you'll remember forever) for basics at least.
Architecture school gave me a sense of design that applies to many areas, photography too.
Books of course, but as you point out, value can be limited.
Sharing work with others, shooting in groups, etc.
Fred Picker's newsletters and phone conversations - again one viewpoint, but you never know where you will pick something up that changes you. Similarly, Lenswork magazine.
Teaching is great, if your students are in the habit of questioning your lessons.
Doing it.
I know you asked for one - I got on a roll.