Do you feel a mild strain when confronted with yet another enthusiastically chanted conviction that an understanding of the 12 principles will manifest the essence of animaiton, like some burning bush mounted next to the Miyazaki figurines on your monitor. Animation is a mysterious art, and it takes more than the established technical guidelines to find your way to true performance.
If you rightly suspect that there's more to good animation than 12 clearly defined principles then the book Animation Insiders is for you. And if you're strapped for cash or a starving artist, well - it's still for you because it's been published as a free eBook at les Shamans under the direction of Patrick Beaulieu.
so... GO GET IT! 
As you know, a gift horse isn't often looked in the mouth. Its easy to recommend a freely offered book as being worth it's weight in pixels. But you shouldn't just get this book, you should read it. Slap it on your slick new tablet and open it often.
Animation Insiders is a collection of essays from various animation professionals and reads as such. They are sometimes loosely structured and informal in tone, sometimes concisely laid out advice documents. What they all share are oozes of experience in navigating the very divergent realities of working as an animatoe, animating and being a person. Yeah... there's stuff about keys and lay-out poses, gimbal lock and euler filters. But also about time management, personal motivation and career expectations. There are even bits about keeping the whole thing in context as an activity within the much larger thing called life.
It's about animation as both commerce and art and ultimately about people. And that's a good thing... because that's what animation is about. So get it... and get it.