I received a question today from a connection at Northwestern, asking about systems thinking resources. She'd already explored classic material from Donella Meadows and was curious about other resources. I don't pretend to have an all-inclusive list, but here are a few from my viewpoint.
I have been working from the Theory of Constraints direction of thinking, and I have been exploring Lean as well. Both disciplines have a lot of specific tools that people usually see first, but they have some deep roots in thinking systematically about problems and the environment in which they happen. A great resource on the TOC side is William Dettmer's The Logical Thinking Process: A Systems Approach to Complex Problem Solving.
On a different angle, the discussions of Dave Snowden and the Cynefin framework are a related way of looking at the world. I'm not sure it is "systems thinking" directly, but the way Snowden talks about his client work sounds very systematic to me.
Of course, there are discussion groups for just about any perspective on systems thinking. I have been lurking in the very active Systems Thinking World group on LinkedIn. And I just happened across Prof. Sam Culbert today via the LeanBlog podcast. His focus is on people / people management, but his background is in systems thinking (and several other disciplines), and he uses that to inform his discussion and thinking.