Throughout the 20th Century, Karl Marx’s visage often appeared on communist banners in the guise of a mighty, bearded prophet, the first and greatest in an array of great thinkers that also included Engels, Lenin, Stalin, and Mao. So it may be slightly odd and amusing to think of him in his mid-20s living in Paris as the editor of a magazine. Digging into the letters and writings that Marx produced in those years, and the writings of those who knew him, always turns up something interesting—and occasionally something utterly fascinating. The year was 1844. Hoping to unite German and...