What psychological or sociological concepts form the basis of Principled Negotiation?
That' a very difficult question because research on negotiations is so varied and multidisciplinary. There is prescriptive research stemming from a decision analysis approach and game theory, descriptive research from behavioral economics, and more process-oriented, social-cognitive research. 

As an excellent starting point into psychological research on negotiation I would recommend Leigh Thompson's book:

Thompson, Leigh. 2013. The Mind and the Heart of the Negotiator. Upper Saddle River: Pearson, 6th ed.