Here are a few of my favorites:
Scholes, Robert (1968), Elements of Fiction (New York: Oxford University Press). [a very short book, really just a long article]
Lewis, David (1978), "Truth in Fiction,'' American Philosophical Quarterly 15: 37-46.
Castañeda, Hector-Neri (1979), "Fiction and Reality: Their Fundamental Connections; An Essay on the
Ontology of Total Experience,'' Poetics 8: 31-62.
Parsons, Terence (1975), "A Meinongian Analysis of Fictional Objects,'' Grazer Philosophische Studien 1: 73-86.
plus, of course,