During the Cultural Revolution, Red Guards started a huge purge that swept the country, 'dragging out' and prosecuting all those ostensibly fitting the description of 'Monsters and Demons' - Niugui sheshen (cow monsters and snake demons). This metaphor was rooted in Buddhist demonology, and an especially potent weapon to demonize one's opponents. All 'evil spirits' could be identified and combated with the 'demon-exposing mirror' (zhaoyao jing) of Mao Zedong Thought, which in this poster is conveniently held up. The book is a volume of the "Selected Works of Mao Zedong" (Mao Zedong xuanji - 毛泽东选集).
Not only people were identified as monsters, all publications that were seen as old and feudal and therefore opposing Mao or his policies were classified as poisonous weeds and destroyed.