There are two classic texts that deal with crowd psychology and the irrational behavior that characterizes large groups of people acting en masse. They are The Crowd, and Extraordinary Popular Delusions and the Madness Crowds. Both books provide lucid and witty insights into the madness of crowd psychology, such as the tulipmania in Holland, when the price of tulip bulbs was up to astronomical heights. Both of these books are combined into a single volume for the price of one!