Thinking in systems - a primer

is an excellent book edited by Donella Meadows (isbn 978-1-84407-726-7). As usual I'm going to quote from a few pages:
The system, to a large extent, causes its own behaviour.
If you see a behaviour that persists over time, there is likely to be a mechanism creating that consistent behaviour.
Economies are extremely complex systems; they are full of balancing feedback loops with delays and they are inherently oscillatory.
This capacity of a system to make its own structure more complex is called self-organization.
Complex systems can evolve from simple systems only if there are stable intermediate forms.
In hierarchical systems, relationships within each subsystem are denser and stronger than relationships between subsystems.
Hierarchical systems evolve from the bottom up.
Systems fool us by presenting themselves… as a series of events…
Side-effects. This phrase means roughly "effects which I hadn't foreseen or don't want to think about."
Everything is connected to everything else - and not neatly.
We have to invent boundaries for clarity and sanity; and boundaries can produce problems when we forget that we've artificially created them.
Economics evolved in a time when labour and capital were the most common limiting factors to production. Therefore economies keep track only of these two factors.
Much of the action you were trying to correct was in response to your own action.
Before you disturb the system in any way, watch how it behaves.
Everything everyone knows is a model.