Two Classics in ONE VOLUME, "The Crowd" & "Extraordinary Popular Delusions.
The first book, THE CROWD by Gustave Le Bon, it is a classic study on mass psychology, explaining why large crowds of people seem to sometimes become emotionally "infected" and function without objective thought. This book studies how such crowds become mobs.
The second book, EXTRAORDINARY POPULAR DELUSIONS AND THE MADNESS OF CROWDS, focuses on a notorious stock swindle and also the Holland tulip mania which drove people to fanatic investments in tulip bulbs, driven by greed.
These are both foundation texts in mob or crowd delusion and psychology. Much of the psychological information remains valid today, and these are useful references for those who would understand popular psychology, and what motivates people to act violently, destructively, or merely foolishly.