The answer to the riddle the Sphinx asked Oedipus is: a man.
A man is a baby in the morning of his life and he crawls on four feet.
A man is an adult in the noon – the middle part – of his life and he walks on two feet.
But when a man is old, in the evening of his life, he walks with a cane, on three feet.
(This would be true of women, too, but the Greeks, being Greeks, told it about men only).
Greek Theatre, by Stewart Ross (1999). Easy reading.
Greek and Roman Theater, by Don Nardo. For teenagers.
Ancient Greeks: Creating the Classical Tradition (Oxford Profiles) by Rosalie F. Baker and Charles F. Baker (reprinted 1997). Short biographies of many famous Greeks including Sophocles.
D’aulaire’s Book of Greek Myths by Edgar and Ingri D’Aulaire. Includes the story of Oedipus.
Greek Tragedy: A Literary Study, by H. D. F. Kitto (reprinted 2002). A classic discussion of the meaning of Greek tragic plays, by a specialist.