The classic pairing that always springs to mind is the Veronica Speedwell series by Deanna Raybourn. The dynamic between Veronica, a fiercely independent lepidopterist, and Stoker, a brooding natural historian, is the engine of the books. They're constantly thrown into investigations involving Victorian-era curiosities and aristocratic scandals. The romance is a fantastically slow-burn element woven through the mysteries; it's less about grand declarations and more about two incredibly sharp, wounded people learning to trust each other. The banter is top-tier, and the plots have a Gothic-tinged feel that's just delightful.
For something with a more modern, procedural feel but still a strong romantic through-line, I'd point to the In Death series by J.D. Robb. It's futuristic, sure, but at its heart, it's about homicide lieutenant Eve Dallas and her billionaire husband Roarke. The first book, 'Naked in Death', sets up their relationship against a gritty murder investigation. What's interesting is that the romance isn't the mystery to be solved; it's an established, evolving partnership that provides a stable center amid the grim cases. The sheer length of the series lets you see a relationship develop over decades, which is pretty unique in the genre.
On a completely different note, Tasha Alexander's Lady Emily series is another historical option. Emily's intellectual curiosity and her relationship with the perceptive Colin Hargreaves drive the narratives. The early books heavily focus on the tension between her desire for independence and the societal expectations of the time, with the romance serving as both a complication and a source of strength. It feels less like a 'detective hero' in a traditional sense and more like an exploration of how a woman in that period would logically engage with sleuthing, often aided by a partner who respects her mind. The romantic plot is integral to her character's growth.