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Most any book on codependency will mention substance abuse, if for no other reason than the concept began there. Treatment of codependency is about empowering the person who is the caregiver to stop that sort of behavior. The best book I can recommend (I'm a LMHC) is Codependent No More. I would also suggest looking at Attachment Theory, as some researchers believe it can encompass the problems ascribed to codependency in a wider-reaching theoretical approach. To that end you might read Attached.
Maybe check out the work of Pia Mellody. It's nearly impossible to speak about codependency outside of the context of addiction. That being said, Pia comes at codependency from the perspective of trauma and attachment
[Please be open-minded since we're all just trying to help here]
Maybe we can try looking at codependency from a different angle. We are made of the physical body, emotional and spiritual elements.
If biology is the logical step towards understanding the pathway of the physical body, then astrology explains the whole gravity of your personality. Not only does it explains who you are, it also explains how you became who you are. One can only heal when there is an awareness of the root of an event. (There are many psychotherapists that are astrologers as well, a faster way to identify the cause of a trauma or event.)
The placements of Neptune and Pluto between two individuals are highly related to codependency and empath/narcissist or victim/saviour relationships. Know your boundaries and make them known, instead quietly resenting yourself for being submissive to the other party.
Healing might take place after reading a book, meeting someone, witnessing an event. What harm does it do to give it a try?