Cassandra was a priestess of Apollo in Troy before the Trojan War. She was very beautiful, and Apollo saw her and fell in love with her. He offered her the gift of prophecy (being able to see what was going to happen in the future) if she would kiss him. She agreed, and he gave her the gift, but when he went to kiss her she spit in his mouth. Apollo was very angry. He could not take away her gift, but he changed it so that she would always know what was going to happen, but nobody would ever believe her when she told them.
Sure enough, Cassandra told all the people in Troy to watch out for the Trojan Horse, but nobody paid any attention. After the Trojans lost the war, the Greek warrior Ajax (not the same one as the other Ajax) attacked Cassandra. She was holding on to a statue of Athena for protection, but Ajax pulled her away from the sacred place and raped her. Then he gave her to Agamemnon as a slave. Agamemnon took Cassandra home to Mycenae, where she warned him that Clytemnestra was going to kill him, but again no one believed her. After killing Agamemnon, Clytemnestra killed Cassandra too.
Like her owner Agamemnon, Cassandra’s showing hubris: she deliberately goes against what a god wants. That’s never a safe thing to do, in Greek mythology.
Cassandra : A Novel and Four Essays, by Christa Wolf (1988). A feminist re-telling of the story of Troy through the eyes of Cassandra. Critically acclaimed (for adults).
The Autobiography of Cassandra: Princess & Prophetess of Troy, by Ursula Molinaro. A similar re-telling (1979).
The Iliad of Homer (Oxford Myths and Legends), by Barbara Leonie Picard. A retelling of the story.
The Iliad (Penguin Classics) by Homer. Translated by Robert Fagles. A great translation!
The Trojan Women and Hippolytus (Dover Thrift Editions) by Euripides. Continues the story of Cassandra, in Euripides’ play about the fall of Troy (translated into English). Very cheap!
The Oresteia, by Aeschylus, translated by Robert Fagles (Penguin Classics). The end of the Cassandra story, and her murder, by the same translator as the Iliad above.