Psychology
Nathaniel Branden. The Six Pillars of Self-Esteem; and Self-Esteem at Work: How Confident People Make Powerful Companies. Branden has written extensively on self-esteem, with the first book being his treatise on the topic, while the second discusses the importance of this topic in a business environment.
Robert B. Cialdini. Influence: The New Psychology of Modern Persuasion. A fascinating book, shedding much light on human behavior.