Erik von Kuehnelt-Leddihn's Leftism is a fascinating read.
The book was first published in 1974: it is a tour de force of left-wing thought. Readers should be aware: this is not a pro-Left book or an objective history. Kuehnelt states openly that he believes in personal liberty and writes from a Christian point of view. In case you were wondering, the author is a Roman Catholic.
Kuehnelt does provide a chronological story of how Leftist thought development. He argues that while 'socialist' ideas have been around for a very long time, what we understand as the Left began with the French Revolution. There were many contributors, de Sade, Marx, the Fabians and Marcus. Decades before Jonah Goldberg made the connection between socialism and fascism, Kuehnelt explains why Mussolini and Hitler are on the Left and not the Right.
Kuehnelt believe that human beings are torn between a desire to subsume their identity by proclaiming connections with others and the impulse towards diversity. He sees diversity creating a world where personal liberty can flourish. If the need for identity wins out there is an inevitable drive towards equality. Mix that with envious hatred and according to Kuehnelt you get an ideology that leads to a desire to change human beings. If anyone refuses to bend to the will of this almost religious ideology, the consequences can be horrific.
Whether you agree with all or some of Kuehnelt's arguments, the book is a thoughtful, well researched work.
The book can be downloaded or bought from the Ludwig von Mises website.