IN THIS later novel by Graham Greene, the author continues to explore moral and theological dilemmas through psychologically astute character studies and exciting drama on an international stage. The title character of Monsignor Quixote is a village priest, elevated to the rank of monsignor through a clerical error, who travels to Madrid accompanied by his best friend, Sancho, the Communist ex-mayor of the village, in Greene's lighthearted variation on Cervantes.