Heaps of Pearl (October Daye, #09.1)

Baron Patrick Twycross was an unlanded, comfortably unambitious man. He knew that one day he might inherit his family's estate, but as that day was hundreds of years and most of a continent away, he was content to spend his time fiddling in his workshop and trying to keep his friend Simon from doing something foolish enough to get himself killed. He was happy; he had no aspirations. So how did he wind up married to a mermaid?
When King Gilad holds a ball in honor of the abdicating Duke of Saltmist and his daughter, the Duchess-in-waiting, there's no real way for the nobles of the Mists to avoid attending. All Patrick is hoping to do is survive the night without utterly embarrassing either himself or Simon, or attracting the wrath of Simon's wife. Alas for him, things are going to get a lot more interesting than he's counting on...
Heaps of Pearl is best read after The Winter Long. It is a historical story, but contains character relationship spoilers through book eight.