Assertiveness Handbook by Randy Paterson

When I Say No I Feel Guilty, by Manuel Smith. Going on 30 years old, but still one of the top sellers. A bit better on business relationships than personal ones. feelings of others.

Your Perfect Right, by Emmons and Alberti. Nice and chatty style, good on the beliefs that hold us back from being more assertive.

Asserting Your Self, by Bower and Bower. A nice, focussed book with an emphasis on making requests of others.

Messages by McKay Davis & Fanning (on general communication) – useful for people with limited social skills trying to learn the gamut of communication strategies from assertiveness to asking for a date.