Maintained by the Human Systems Dynamics Institute (HSD), this Wiki gives a great introduction to many of the central topics pertinent to systems thinking.
On the main website, the Institute offers a free white paper every month. The latest, “Attend to the Whole, the Part, and the Greater Whole”, does a fine job of introducing issues of scalability in nested systems, and will only available until August 15, 2013. Get it while it’s hot.
BTW- Signing up for the basic HSD community is free…There is a charge if you become an “Associate”.