Mysticism Defined by Rudolf Otto
- A brief excerpt on numinosity from his The Idea of the Holy.
- http://www.bodysoulandspirit.net/mystical_experiences/learn/experts_define/otto.shtml

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.
- http://www.friesian.com/edwards.htm

Biography.com: Otto, Rudolf
- Concise biographical information.
- http://search.biography.com/print_record.pl?id=10214

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.
- http://www.friesian.com/otto.htm