Mr. Lewis himself regarded "Till We Have Faces" as one of his best books. But it's hard to understand its full meaning without knowing his other work. Anyway, here is the basic premise:
In ancient times, before the Roman Empire existed, there is a small kingdom somewhere in what we know as Eastern Europe--possibly in modern Yugoslavia. It is not IN Greece, but is close enough to Greece to be influenced by Greek civilization. The king of this kingdom dies with no son to take over the government, so his eldest daughter Orual, a woman both intelligent and physically strong, assumes the rule. She succeeds in making the small nation more powerful and prosperous, but is bitter against the gods for denying her many of her personal wishes in life. In the end, the reader is made to understand that Orual's complaints against "the gods" reflect the way modern people get angry at the actual God.
If you can find a good description of the Greek myth of Psyche, it will help you to follow the more mysterious parts of this novel.