Music Theory Fast
I have been delivering a crash course in ABRSM Grade 5 Music Theory this week, for some nice people in Stamford (UK). They called their course Music Theory Fast, and whilst I cavilled, of course, at the grammar, I liked the double meaning of "Fast": we did it quick, or rather quickly, and hopefully it will stick fast.
I had thought, as I was preparing for the course, that I would collate and write lots of content and exercises and then publish them online, but I am pleased to say that I have been beaten to it.
MyMusicTheory offers the best online course and materials I could find online, and probably better than the books. Amazingly, their resources are free if you don't need to print, but please follow my lead, and buy them. They deserve support for all their hard work, and their course is thorough, and well thought out.
P.S. I have no connection with MyMusicTheory.
Update 2020 - I have written my own materials, but not published them. I run the course regularly, and it has its own website at Music Theory Fast.