Some more backround information about the NKVD regime you can find here:

• Memorial (Russia)
  
  
• Encyclopaedia Britannica
  
  
• Literature
  
  
  • Dictionaries and Lexica
    
    
  • Soviet Laws
    
    
  • General Publications
    
    
  • Memoirs and Eyewitness Reports
    
    
  • Special Researches and Publications
    
    
  • Eyewitness Reports about Solovki
    
    
  • Unsorted English Publications
    
    
  • Unsorted German Publications