On Ch. 4 of Lewis's book Counterfactuals (1973) and the essays “Scorekeeping in a Language Game” (1979) and “Truth in Fiction” (1978).
What makes a sentence about possibility true? Lewis things that we need possible worlds that really exist in order to make sense of our modal intuitions. He uses this possible world talk to make sense of conversations and the worlds created by fiction writers. With guest Matt Teichman from Elucidations.