I had a read through all of the workbooks as I hadn't seen them all before. Chapter 1 was worthwhile as it defines assertiveness in terms of your rights.
I also found chapter 4 particularly useful, and maybe should have been put before chapter 3 if an order was intended. It gives a few techniques for being assertive with people, explains them, their situational advantages and provides examples.
Chapter 5 has a good verbal description of a full body relaxation technique that I've run through with a couple of psychologists previously. You tense, then relax parts of your body progressively with your eyes closed.
Chapter 7 on responding to criticism assertively was was worthwhile for me. It covers responses for criticism that you think are valid or non-valid, including ones that can get you out of a conversation or help defuse it, if telling the truth would make them more aggressive.
Chapter 8 on dealing with disappointment was nice and brief, and seems like a reasonable checklist of questions to ask yourself to make sure ,if you're feeling disappointment, it's within reason.