Chances are you’re thinking at some point, “What is a tritone sub?*” or “How do I know what notes are in a D symmetrical diminished scale?**” *** Some of these concepts might be explained on my site, but I highly recommend picking up one [or two] [or all??] of these things to get a grip on the simpler stuff. This page is always getting updated, though!
These posts will jump in assuming you have a decent level of theory [you understand scales, intervals, basic rhythms, a concept of irregular rhythms, and traditional harmony up to secondary dominants]. If you’re scratching your head at some of this, look into these.
The Jazz Theory Book Mark Levine’s greatest gift to the world, so to speak. It says Jazz, but it should really say Modern. It’s pretty applicable to everything, though it does have a jazz heavy approach. Don’t be daunted by that – it starts you odd at the absolute beginning and before you know you’ll be shredding Coltrane changes upside-down. [Or not.] Book or eBook.
Teoria. Pretty nice site to start/continue your theory adventure, and it makes a more traditional approach. Consider this – it also covers some of the odder things. It’s been around for literally 20+ years so it’s gotta mean something.
There’s a ton of theory sites out there as well that are useful. Use an approach that works for you!
Weeb Theory 