Aristotle – The Poetics
A shorter work with useful insights for appreciating storytelling in general, and Greek drama and poetry in particular. Aristotle offers a key insight for making a character believable: a character’s every action and every word should be based on either necessity or probability.
The plot necessitates some actions on the character’s part. What the audience knows about the character’s personality dictates the probability that his reactions are believable. To use a lowbrow example, the reason it’s so funny when Homer says something intelligent on The Simpsons is that is so out of character.
Some of Aristotle’s other ideas about what makes good drama or good poetry seems to be his personal taste. This being subjective, it need not be taken as gospel.