In this article “A Level of Development Specification for BIM Processes” Jim Bedrick explains the evolution of LODs and in particular the fundamental LOD definitions.

http://www.aecbytes.com/viewpoint/2013/issue_68.html

The BSI document “PAS 1192-2:2013 Specification for information management for the capital/delivery phase of construction projects using building information modelling” can be freely downloaded from the bsigroup website.

A small regsitration is required, but the PDF is free of charge:

http://shop.bsigroup.com/Navigate-by/PAS/PAS-1192-22013/