My favorite type of book is not a "mystery" but a book about PEOPLE solving a mystery, and people, at least the type of people worth making into protagonists, seldom navigate life for long without at least a dash of romance. I only discovered this series last week and am currently reading book 6, I think. To me, the whole series is the story of Sarah and Frank and how they solve mysteries together, and the books are really just chapters. Their relationship, and how it evolves and affects each of them is central to how they work together. I don't even remember individual titles because it is just one long story to me. And just to be clear, it is a fascinating, thoroughly enjoyable story!