The Tide Knot by Helen Dunmore was just released in the U.S.
Description: Sequel to Ingo. As Sapphire and Conor explore further into the kingdom of the Mer, they learn of a threat growing deep beneath the surface of the sea, where the wisest of the Mer, Saldowr, guards the Tide Knot. Ingo is restless, and soon both Sapphire and Conor will have to answer the call of the Deep.
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