The Timeless Way of Building
Alexander, C. 1979. The Timeless Way of Building. New York, NY, USA: Oxford University Press.
This source is considered a primary reference for the Patterns of Systems Thinking article.
This source deals with systems pattern. Significant emphasis is placed on the on the pattern of reconciling and resolving competing forces, which is an important application of the "yin yang" principle discussed in Patterns of Systems Thinking.