Here's another good book recommendation: Boundaries by Dr. Henry Cloud and Dr. John Townsend.
In order to call themselves good Christians, many people have drawn overly flexible boundaries (unwilling to say no, always accommodating others' needs) or overly rigid boundaries (to the point of being righteous and judgmental). Psychologists and inspirational speakers Cloud and Townsend show readers how to set reasonable boundaries in order to follow the true path of Christianity.
Having clear boundaries is essential to a healthy, balanced lifestyle. A boundary is a personal property line that marks those things for which we are responsible. In other words, boundaries define who we are and who we are not. Boundaries impact all areas of our lives: Physical boundaries help us determine who may touch us, mental boundaries give us the freedom to have our own thoughts, emotional boundaries help us to deal with our own emotions and spiritual boundaries help us to distinguish God's will from our own.
This book uses Christian-based teaching to show you how to set proper boundaries with yourself and others in your life. I highly recommend this book if you are someone who "gives in" to others too much or has too many "walls" preventing proper boundaries with others.