See Chapter 49: Jorge E. Viñuales, ‘Sources of International Investment Law: Theoretical Foundations of Unruly Practices’ and chapter 50: Stephan W. Schill, ‘Sources of International Investment Law: Multilateralization, Arbitral Precedent, Comparativism, Soft Law’
The Oxford Handbook on the Sources of International Law by 
International Investment Law: the sources of rights and obligations by