To Kill a Kingdom is a romance (marketed as general YA) between two killers. Princess Lira is a siren. She kills a human prince and steals his heart every year on her birthday (hence the heart collection). Prince Elian is a human prince who sails the seas with his loyal crew, searching for and killing sirens (because of all the murders).
The descriptions in the book are amazing and brutal. The world is divided into cities. There’s Midas, the city of gold, whose golden buildings cast reflections on the water so that even the sea looks golden. There’s Eidyllio, a land dedicated to romance and covered with flowers. There’s the terrifying land of Kleftes, where thank goodness we do not spend any time – it’s where thieves and slavers take refuge. Of course there’s also the undersea world, which is not described as much visually but which is described in terms of violence, politics, family, and class barriers (sirens can’t stand mermaids).