If you've always wanted to learn more about song writing and the structure of music, this free online book is filled to the brim with pages that make understanding music theory easy. Toby W. Rush's "Music Theory for Musicians and Normal People" covers a massive range of topics, including pitch, rhythm, scales, intervals and harmonics.
The online book itself is more arranged as a collection of high-quality PDFs that offer diagrams, notes and tips for everything related to music theory. There's around 50 PDFs to go over, but each one could make for a useful reference printout if there was a specific topic you're interested in. Learning to play music can have a number of benefits, and although music theory isn't required for doing so, it can certainly help if you want to create your own tunes. To see the complete collection of Rush's graphics, check out the link below.
Music Theory for Musicians and Normal People [University of Dayton]