Here is a thread I created on another forum:
https://www.thegearpage.net/board/index ... c.1371119/
It's really aimed at all the "popular music" guitarists on that forum who often have learned to play by ear or tab, but who've decided they need a little more theory background.
Honestly, as a university instructor who both studied and taught theory, I'd say your best bet is to find a course for non-majors - a "basic musical skills" course and take that.
Trying to "self learn" Music Theory is a little bit of an exercise in futility.
There's also a lot of misunderstanding out there about what music theory is, what it's supposed to do, and how you "use" it.
Almost all of the world class level Classical musicians I've met (I ran a concert hall) did not have as strong a grasp of music theory as a 2nd semester college student (that was getting an A...). Most of them took music theory in college or had it as part of their tutelage (private lessons, some conservatory studies, etc.) but most of them had forgotten most of it as this stage in their career.
IOW, they know enough theory to "get by" and that's it.
On a Classical Guitar forum, those interested in playing Classical Guitar as a serious avocation, I'd say you need to concentrate on your playing, and do that by studying with an expert in the field. You should get whatever theory you need along the way.
You can self study it for your own edification, but it's not necessarily going to make you a better player of Villa-Lobos or something.
Now, if you're interested in pop styles, there are probably more practical uses of music theory, such as what is needed for improvisation in various styles and for composition and/or arranging, etc. But even then plenty of people learn that stuff "on the streets" instead of through any structured study.
I could recommend the books we use in college, but without guidance of some sort, I'm not sure how beneficial it would be. Besides, very few musicians have PhDs in music theory - most of them are in a non-music field and "basic theory" is all most music majors get.
So, how much do you know already, and why do you want to learn more?