When I worked on a crisis hotline, reading anything which could help me answer the calls better was counted as "productivity", so I read a lot! Here are a few of the books I consider 'must have' references for communication tools.

• Conversationally Speaking by Alan J. Garner (1981)
• When I Say No, I Feel Guilty by Manuel J. Smith (1985)
• The Dance of Anger by Harriet Goldhor Lerner (1985)
• The Gentle Art of Verbal Self Defense by Suzette Haden Elgin (1980)
• Tongue Fu!: How to Deflect, Disarm, and Defuse Any Verbal Conflict by Sam Horn (1996)