One Salt Sea (October Daye #5) by Seanan McGuire
Synopsis from GoodReads:
October “Toby” Daye is finally doing all right—and that inevitably means it’s time for things to take a turn for the worse. Someone has kidnapped the sons of the Duchess Dianda Lorden, regent of the Undersea Duchy of Saltmist. To prevent a war between land and sea, Toby must not only find the missing boys, but also prove that the Queen of the Mists was not behind their abduction. She’ll need all her tricks and the help of her allies if she wants to make it through this in one piece.
Toby’s search will take her from the streets of San Francisco to the lands beneath the waves. But someone is determined to stop her—and whoever it is isn’t playing by Oberon’s Laws. As the battle grows more and more personal, one thing is chillingly clear. When Faerie goes to war, not everyone will walk away.