What should I read to learn about decision theory?

Abram Demski and Scott Garrabrant's post on decision theory provides a good overview of many aspects of the topic, while Functional Decision Theory: A New Theory of Instrumental Rationality seems to be the most up to date source on current thinking.

For a more intuitive dive into one of the core problems, Newcomb's problem and regret of rationality is good, and Newcomblike problems are the norm is useful for seeing how it applies in the real world.

The Alignment Forum tag for decision theory has lots of additional links for people who want to explore further.