I recently found a textbook on music theory which is published under Creative Commons license. So in terms of bang for buck, it won’t get any better.
I’m not that deep in already, but it is very approachable imo, with loads of links to exercises and growing in complexity as well as it easy to skip over known topics.
Only downside is, that its not bass focused and probably requires a piano, midi controller or other key instrument to really get the most out of it.
And I don’t know how this compares to other music theory books, so take my enthusiasm with a grain of salt.
Allow me to share your enthusiasm! This looks great! A couple of points from the intro that jump out at me:
Unlike textbooks from big name publishers which cost hundreds of dollars, OMT is completely free to use.
OMT takes full advantage of the online and web-based format and includes audio and video examples throughout.
Many chapters include interactive content that allow students to easily test their knowledge on a concept and immediately find the correct answer, facilitating self-teaching.