The Black Moon by Winston Graham
The Black Moon is the fifth novel in Winston Graham's hugely popular Poldark series, which has become a television phenomenon starring Aidan Turner.
Cornwall 1794. The birth of a son to Elizabeth and George Warleggan serves only to accentuate the rift between the Poldark and Warleggan families. And when Morwenna Chynoweth, now governess to Elizabeth's eldest son, grows to love Drake Carne, Demelza's brother, the enduring rivalry between George and Ross finds a new focus for bitter enmity and conflict.
The Black Moon is followed by the sixth book in the Poldark series, The Four Swans.