“Materials of Music”, by Robert Reno, has been one of the best music theory textbooks available online to teach you basic and advanced music theory concepts.
Bob created a book as a college level teaching text for both those that have had experience and those with minimal experience to music. The Materials of Music is no longer available on our site.
However you can check out his new offerings here including Foundations and Tonal Harmony:
He does a fantastic job of breaking down each of the materials of music from the basics of music theory to the advanced concepts on harmony.