"The Crowd: A Study of the Popular Mind" is a book written by Gustave Le Bon in 1895. It explores how individuals in a crowd can exhibit behaviors and emotions that differ from their actions when alone. Le Bon argues that crowds can lead to irrationality and a loss of personal responsibility, as individuals become influenced by the collective mindset. The book also discusses the psychological mechanisms that drive crowd behavior, such as suggestion and contagion. Le Bon's work has influenced various fields, including psychology, sociology, and political science, by providing insights into group dynamics and mass movements.