System of Systems Engineering – New Challenges for the 21st Century
Jamshidi, M. 2008. "System of Systems Engineering – New Challenges for the 21st Century." IEEE Aerospace and Electronic Systems Magazine. 23(5) (May 2008): 4-19.
This source is considered a primary reference for the Groupings of Systems article.
This article is from one of the leading experts in systems of systems engineering. It provides a good overview of the state of the art in engineering SoS and the challenges that need to be addressed.