Kurt Vonnegut - The Shapes of Stories. Stumbled across this on a Google for something completely unrelated, but definitely going to use it to help frame the way I look at stories. And definitely re-reading Slaughterhouse Five sooner rather than later.
The main rule of writing is that if you do it with enough assurance and confidence, you're allowed to do whatever you like. So write your story as it needs to be written. Write it honestly, and tell it as best you can. I'm not sure that there are any other rules. Not ones that matter. ~ Neil Gaiman