I got turned onto this book from John K's blog, and it does not disappoint. The book teaches the importance of construction in animation and it's helped me a great deal in my general drawing also.
The majority of this book can be found in his other book though, Cartoon animation, which I got after reading this and seeing how good it was. So if you want to get the other book, that one does have this stuff in however it has very slightly different illustrations.
I personally prefer this one though, it's short simple and it's full of character.