Conversation so far:
What is convention?
David Lewis's first monograph was Convention: A Philosophical Study
What year was David Lewis's first monograph Convention: A Philosophical Study written?
in 1969
Did David Lewis  monograph Convention: a philosohical study win any awards?
David Lewis's first monograph Convention: A Philosophical Study won the American Philosophical Association's first Franklin Matchette Prize for the best book published in philosophy

Question: What was his monograph about?
Self-contained rewrite: What was David Lewis monograph Convention: a philosophical study about?