"The Assertiveness Workbook: How to Express Your Ideas and Stand Up for Yourself at Work and in Relationships" by Randy Paterson is the best I've read, although

"The Assertiveness Workbook: How to Express Your Ideas and Stand Up for Yourself at Work and in Relationships" by Randy Paterson is the best I've read, although "Crucial Conversations" is very good as well. He discusses the three types of barriers (Stress, Social, and Belief) to assertiveness, and offers specific exercises to work through them. He's also very realistic - for example, he mentions that some relationships will become temporarily more difficult once you start asserting yourself more, but you shouldn't be deterred.