Thomas Harris was an American psychiatrist who authored the bestselling self-help book "I'm OK, You're OK" in 1969. The book introduced concepts from Transactional Analysis including ego states, life positions, and transactions between people. It describes how people operate from parent, adult, or child ego states and how their interactions can be understood through concepts like positive and negative stroking. The book was influential in popularizing these ideas and becoming a bestselling manual for understanding human behavior and relationships.