Music theory puts some people off terribly; it's all charts and scales and practice and effort and a big load of arse which is mostly irrelevant to someone who wants to bang out a few tunes. Some people are openly hostile to symbols such as bVIma7-I and phrases such as dominant seventh sharp ninth.Download: The Ravenspiral Guide
But the thing is that i'm a tune-banger myself and would rather actually write music than read about writing it. So my sympathies are very much in your corner if that's what you're like. I'm not going to try to lie to you and say that this guide doesn't have any charts or scales, but that's where the similarities between the Ravenspiral guide and Composing for the Modern Music Student end.
First off, music theory might not even be that useful to you. If you've never been stuck for any length of time at any moment of writing music, good for you. Always known what notes come next? Happy with your tunes? Great. Go do what you do and all the best of luck to you; may you be the next Aphex Twin. This book probably won't teach you anything you need to know and may well ruin the things you've already picked up.
It's getting harder and harder to find some nice tutorials, guides about music production. But luckily, there are still some resources ;) Ravenspiral Guide is a freely available online resource for those who want to know a bit more about music theory. The guide is in PDF format, has 92 pages but it's still, as author - Simon Bennet states, unfinished just like the art of music itself :)