Thinking in Systems, Donella H. Meadows, Chelsea Green Publishing. (2008)

Donella H. "Dana" Meadows died in 2001, the draft of this book was completed in 1993.  The draft was edited and published by Diana Wright in 2008.  This book contains a distillation of the work initiated at MIT by Jay Forrester and the MIT System Dynamics group.  Systems thinking is applicable in many areas.  This may well be the best introduction.