Why does the poet compare Helen to the cover of a good ancient book?

Because Helen has always been the source of inspiration for many western writers.

In the long history of literature dominated by male-dominated discourse, she has always been the object of worship or accusation.

Helen, the daughter of Zeus and Leda, the third generation king of gods in ancient Greek mythology, grew up in the palace of her late father Tindarius, king of Sparta. She is the most beautiful woman in the world. When she was born, God gave her the ability to imitate any woman's voice. When she grew up, she eloped with Paris, the prince of Troy, which triggered the famous Trojan War.