"The Millionaire Next Door" is a book by Thomas J. Stanley and William D. Danko that explores the habits and characteristics of wealthy individuals in America. The authors conducted extensive research to identify common traits among millionaires, revealing that many live frugally, prioritize saving, and invest wisely rather than flaunting their wealth. The book emphasizes that true wealth is often hidden, as many millionaires do not display their riches through luxury items or extravagant lifestyles. Instead, they focus on financial independence and long-term planning, challenging the stereotype of the flashy millionaire often portrayed in popular culture.