The Authoritarian Spectre, by Robert Altemeyer. Altemeyer is a social psychologist at the University of Toronto. The book as a whole is excellent, but there is a chapter in particular on how people read Holocaust denial literature that is especially unsettling—and bears close reading.
Faces of the Enemy: Reflections of the Hostile Imagination, by Sam Keen. Keen was, when he wrote this book, a contributing editor to Psychology Today. The book pretty thoroughly addresses the “psychology of enmity”—and has lots of interesting discussions of propaganda posters etc.
The Weimar Republic Sourcebook (edited by Anton Kaes et. al.). This book, though not devoted to “explaining” the psychology of authoritarianism, is nevertheless highly suggestive concerning the circumstances under which it can thrive. The book consists of source documents from the Weimar Republic period, and there are many resonances in them with things going on today. In other words, it has a surprisingly contemporary feel (which is a depressing thought to contemplate).