Books like The Assertiveness Workbook
Randy J. Paterson
The Assertiveness Workbook is 206 pages, about 3 hours 47 minutes at 250 words a minute.
8 books to read next
How long each takes
Don't Say Yes When You Want to Say No 5.3 h 
When I Say No, I Feel Guilty 5.9 h 
Daring Greatly 5.3 h 
Your Perfect Right 4.2 h 
The Gentle Art of Verbal Self-defense 5.7 h 
Asserting Yourself 4.5 h 
The Complete Idiot's Guide to Assertiveness 6.2 h 
Don't Be Nice, Be Real 5.8 h Bar height shows length. Estimated at 250 words a minute.