Great book with lots of tips and exercises to help you become more assertive. These self help books are great but you need to actually spend the time and do the exercises.
The most helpful part of the book is the DESC Scripting chapters. This is a way to help you deal with conflicts and negotiate a plan with your so called 'downer.' Describe the offensive behavior objectively to your downer. Express how you feel about that behavior in a positive way, use metaphors. Specify how you'd like to see that behavior changed. One at a time and be specific. Spell out the Consequences of such a change and emphasize the positive ones.
Sample scripts are provided and are very helpful. There are appendices dealing with the principles of behavior change, progressive relaxation, and rules for contracting. If you need some help speaking up more with your friends, family, or at work, you'll benefit from reading this book.