The Book of No by Susan Newman. McGraw-Hill; 1 edition (November 15, 2005) As a psychotherapist, I'm continually helping bolster people who have trouble saying 'no.' It's an essential life skill, because if you can't say 'no' when you need to, then your 'yes' is meaningless, and will result in resentment that leaks out in passive/aggressive ways.
Susan Newman's book is a great resource for my clients. It's a reference book for how to say 'no' in a variety of situations and circumstances. Having a script for how to say no encourages people to try it, and they then learn by doing.