Open Music Theory is a natively-online open educational resource intended to serve as the primary text and workbook for undergraduate music theory curricula. OMT2 provides not only the material for a complete traditional core undergraduate music theory sequence (fundamentals, diatonic harmony, chromatic harmony, form, 20th-century techniques), but also several other units for instructors who have diversified their curriculum, such as jazz, popular music, counterpoint, and orchestration. This version also introduces a complete workbook of assignments.
I'm using the book as consulting material so I haven't read it entirely, but I appreciate the amount of effort and detail given to every chapter and book structure. Coming from an intuitive approach into the theoretical realm isn't easy for me but it has helped set some very needed foundations to rest my mind on while playing in order to let my brain focus on the sounds I want. Worth it.