The Political and Social Doctrines of the Unity of Czech Brethren in the Fifteenth and Early Sixteenth Centuries. By Peter Brock. (Slavistische Drukken en Herdrukken, Hoogleraar te Leiden, xi). Pp. 302. 's-Gravenhage: Mouton; London: International University Booksellers, 1957. 47s. 6d.