Damn, I wish I could answer this. I had a fantastic reference text for a course at university, but sold it when I was short on cash. All I can remember is that it was blue. :(

As for other sources, I've used the Chicago and Strunk & White, but neither strike me as ideal for everyday writing.

I'm not sure how instructional it is, but you might want to check out Eats, Shoots & Leaves -- I remember hearing raves about it.