For the parts of microeconomics and game theory which abut on the decision theory discussed here, I would suggest some of these free online textbooks. Registration required, but is apparently fairly painless - even spam-free.
ETA: One branch of microeconomics that I think is particularly important is "information economics". In the course of Googling to find resources on this topic, I ran into this useful encyclopedia.
I would like to learn more about economics but I don't know where to start. Can lesswrong suggest specific areas of economics that are particularly useful for understanding and optimising the world? Specific suggestions such as reading lists and resources would also be much appreciated.