So I've been playing guitar (mostly fingerstyle) for a number of years, and I'd consider myself to be a fairly intermediate player. But I want to start learning some classical theory to (hopefully) really advance and hone my skills, and really just to become more "fluent" with the guitar if that makes any sense. I have next to no classical training previously and one on one lessons aren't an option for me right now, so I'm just wondering if anyone has any advice on where I would begin? Kinda vague I know, but any advice is appreciated! Thanks
"Music in Theory and Practice" is a pretty standard text for use by new classical guitarists.

The website www.justinguitar.com has some good resources as well.

For a great book on theory, although not specifically in relation to "classical music", check out "The Jazz Theory Book" by Mark Levine. It's a classic
My God, it's full of stars!