One great adult romantic mystery story is 'Gone Girl' by Gillian Flynn. It's a story full of twists and turns. The relationship between the two main characters, Nick and Amy, is complex and mysterious. Another one could be 'The Silent Patient' which also has elements of romance intertwined with a deep mystery about a woman who stops talking after shooting her husband.
I would suggest 'Rebecca' by Daphne du Maurier. It has a strong sense of mystery with the new bride trying to figure out the secrets of her husband's first wife, Rebecca. And there are also romantic elements as she navigates her relationship with her husband in the midst of all the mystery.
Sure. 'The Girl on the Train' is an adult romantic mystery. It follows the story of Rachel, who witnesses something strange from the train window. There's a love triangle aspect in it as well as the mystery of a missing woman. The relationships in the story add to the overall intrigue.