Uncertain Summer by Betty Neels

When her fiancé jilted her, Serena gave up all hope of ever getting married. She was determined to be sensible about the whole thing; that was, until Gijs suggested that she marry him.

She liked Gijs very much, and she knew he was fond of her. Companionship seemed as good a basis as any for a marriage—who needed romance anyway? But it turned out Gijs was in love, and Serena couldn’t work out who the lucky woman was.