Open Music Theory Version 2 (OMT2) is another free, interactive online text book, the 2nd iteration of Open Music Theory
Open Music Theory Version 2 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.
Contributors to the online text book are also active in the music theory academic and online “public music theory” communities: Mark Gotham, Kyle Gullings, Chelsey Hamm, Bryn Hughes, Brian Jarvis, Megan Lavengood, and John Peterson
Here’s a screenshot of one of their lessons on borrowed-chord chord progressions:
It’s a perfect, free way to start learning core music theory.