Biography.com: Otto, Rudolf: Concise biographical information.
Friesian School: Rudolf Otto: An account of Otto's relation to Kantian-Friesian thought, and the encumbrance which that rationalistic tradition may have been for him.
Mysticism Defined by Rudolf Otto: A brief excerpt on numinosity from his The Idea of the Holy.
Rudolf Otto in Rem B. Edwards: Kelley L. Ross uses this modern writer as a case study in typical contemporary treatments of Otto's theory of numinosity.