Freud wrote this book in 1921, and its themes are related to his books Beyond the Pleasure Principle-First Edition text and The Ego and the Id - First Edition Text.
Here are some representative quotations from the book:
"(I)n a group the individual is brought under conditions which allow him to throw off the repressions of his unconscious instinctual impulses. The apparently new characteristics when he then displays are in fact the manifestations of this unconscious..."
"Since a group is in no doubt as to what constitutes truth or error, and is conscious, moreover, of its own great strength, it is as intolerant as it is obedient to authority. It respects force and can only be slightly influenced by kindness, which it regards merely as a form of weakness."
"But we do not separate ... one the one hand, self-love, and on the other, love for parents and children, friendship and love for humanity in general, and also devotion to concrete objects and to abstract ideas. Our justification lies in the fact that psychoanalytic research has taught us that all these tendencies are an expression of the same instinctual impulses; in relation between the sexes these impulses force their way toward sexual union, but in other circumstances they are diverted from this aim..."
"From being in love to hypnosis is evidently only a short step. The respects in which the two agree are obvious."
"The group ... agrees with hypnosis in the nature of the instincts which hold it together, and in the nature of the replacement of the ego ideal with other individuals, which was perhaps originally made possible by their having the same relation to the object."