Rudolf Otto (1869-1937)  
   Resource page including biographical and extensive bibliographical information. Also features several online texts in both German and English.
   http://www.netrax.net/~galles/

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

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

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

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