If you are embroiled in high conflict situation, with access interference and alienation, Dr. Richard Warshak's Divorce Poison is an excellent book. Link to Dr. Warshak's website.
Warshak, Richard A. (2001). Divorce poison: Protecting the parent-child bond from a vindictive ex. New York, NY: HarperCollins
To preview the book, go to Dr. Warshak's website using the link above and click on the image of the book. That will take you to Amazon.com and you will be able to review the publishing details, the table of contents, the introduction, the index, and the back cover of the book.
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Garrity, C. B. & Baris, M. A. (1994). Caught in the middle: Protecting the children of high-conflict divorce. New York, NY: Lexington Books. link to cover, Contents, and Introduction)
This book is written for both parents and mental health professionals. It introduces the idea of parenting coordination. Since writing Caught in the Middle, Garrity and Baris collaborated with some others to write Working With High-Conflict Families of Divorce: A Guide for Professionals, which is written specifically for mental health professionals, but would be useful for parents who want to get a detailed understanding of what parenting coordination is.
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If your conflict is generally less extreme, and you just generally find your 'ex' impossible to deal with, Joint Custody With a Jerk: Raising a Child with an Uncooperative Ex, is a useful book. (link to cover, Contents, and excerpts) While the book is a useful resource, we don't condone referring to someone as a "jerk", even if they are behaving badly. It is important to focus on the behaviours, and separate them from the person. If there are mental health issues involved or substance abuse issues, you've probably already learned to separate the person 
from the illness/addiction. Turning specific behaviours into global characterizations is not useful, and when it is a negative characterization of a parent, it hurts the children.
Even if you've never said something like this to your children, or when they were around, if you are thinking it, saying it under your breath, or saying it to other people, your children will pick up on it as strongly as if you said it directly to them.
(If you have a gender neutral image, or an opposite gender equivalent to provide balance, please contact us. The above cartoon is from Father and Child Reunion; How to Bring the Dads We Need to the Children We Love, by Warren Farrell, Ph.D., page 24.)