[REALiSTiC]

Systems Thinking, mainly through reading Russell Ackoff's books. The Art of Problem Solving is good, Turning Learning Right Side Up is eye-opening (and definitely one of my favorite books lately) and currently reading Redesigning Society. Highly recommended.

[raleighm]

I recommend Systemantics also. https://en.wikipedia.org/wiki/Systemantics

[phatle]

I'm reading this subject too. Thinking in Systems: A Primer is also a good book. Highly recommended.

[REALiSTiC]

I started it couple of months ago, but couldn't get past the first few chapters. It was too dry for my taste. Even though she's a brilliant scientist (I'd recommend checking her classic The Limit to Growth), it seemed like the earlier resources about the topic are much better explained rather than the newer stuff.