Butterfly,
Conquering Codependency and Shame probably helped me more than any other books. It really broke down the roots of Codependency and made them easy for me to understand.
Also, there is a lot of information out there on Schemas of codependency. It splits it into a lot of categories (many of us have different umm...symptoms) and then goes even further into where each 'symptom' can come from.
So yeah - like Anvil said
THAT simple
and THAT complex.
(((HUGS))) I burned myself out a little on reading about the background, but it also helped me SO MUCH.