While neither of the following books are technically romance novels, they both have strong romantic relationships at their centers. They're also both well-researched historical novels, so you might accidentally learn something while reading. I know I did.
A Race to Splendor by Ciji Ware has a beautiful cover. It features a woman in a yellow gown set against the San Francisco skyline circa 1906. Yup, it's an earthquake novel. Or, really, a rebuilding-the-city-after-the-earthquake novel. It also leans more heavily toward historical fiction rather than romance, but there's a great sparking-and-spatting relationship that drives the plot.
Amelia Bradshaw returns to San Francisco after her father's death to lay claim to the Bay View Hotel, which she (kinda) inherited from her grandfather. Her father, however, being one of those wastrel fathers so common in Romancelandia, has gambled away the hotel to one J. D. Thayer, who intends to turn it into a den of vice. Amelia goes to work for architect Julia Morgan. The earthquake happens. Amelia winds up working with Thayer to rebuild the Bay View and get the hotel opened before Ms. Morgan finishes work on the Fairmont. Somewhere along the line, Amelia and Thayer fall in love, but, because this is historical fiction and not romance, the Happily Ever After isn't guaranteed. The romantic tension is played out very well, though. Additionally, Ms. Ware does a superb job of detailing the devastation of the quake and its impact on the citizens of San Francisco. Her depiction of the plight of the many Chinese immigrants in the city is especially heartbreaking. It's a novel with a lot going on, but Ms. Ware ties everything together beautifully and delivers a novel well worth reading.
Vienna Waltz by Teresa Grant isn't a romance, either; it's a mystery. But, it has a really strong romantic component ? this time between a husband and wife who are no longer in love (if they ever were). Suzanne finds her husband, Malcolm, in the room of a murdered Russian princess. Circumstances conspire to force them to work together to discover the identity of the murderer, and secrets aplenty are unearthed before they finish. Along the way, they learn more about each other, and, in the end, their marriage is well on track to becoming a love match rather than just a practical arrangement. Like A Race to Splendor, Vienna Waltz uses real events as its backdrop ? in this case, the 1814-15 Congress of Vienna ? and real people as supporting characters.
So, okay, these novels are probably not what you'd want to use as reference materials if writing a paper on either the San Francisco Earthquake or the Congress of Vienna, but they're a lot more engrossing than my high school history books ever were.