The Mermaid Summer by Millie Hunter (Good, 1990, Pbk, 118 pgs,, Harper Trophy)

In good condition: cover has wear and is taped; pages have light usage wear; inside is clean; pages are lightly age-toned. ISBN 0064403440

About a hundred years ago, they say, a mermaid ruled the cold sea that washed around northern lands

Everyone in Jon and Anna's village knows of the mermaid's beauty -- and her cruelty -- and is careful to speak well of her. Everyone but their grandfather, Eric, until the day the mermaid smashes his fishing boat against the rocks.

After that, Eric bids his family good-bye, for no one will sail with him again. But Jon and Anna never stop missing him, so they decide they must defeat the mermaid's curse and bring their grandfather home.