‘Music Theory for Musicians and Normal People‘, by Toby W. Rush, is a collection of PDFs which can be downloaded individually or as a set.
I love these!
The topics range from basic introductory concepts to things that are (in my humble opinion) much more specialised.
You can check out the whole set at http://tobyrush.com/theorypages/
And because I personally appreciate reading about music theory in such an easy to follow format, I’ll give Toby an extra plug: “Each and every one of these pages is available is an 18″ x 24” poster. Buy one or collect them all at the Toby W. Rush Online Store! You can also find a theory-related shirts, hats, bags, buttons and stickers at the T-shirts And Other Stuff page.”