One Salt Sea (Toby Daye Book 5‪)‬

The fifth instalment of the highly praised Toby Daye series.
October ‘Toby’ Daye is finally doing all right – and inevitably that means it's time for things to take a turn for the worse. Someone has kidnapped the sons of the Duke and Duchess 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.
But someone is determined to stop her.
From the streets of San Francisco to the lands beneath the waves, Toby will need all her guiles and the help of her allies if she wants to make it through this in one piece. As the battle grows more and more personal, one thing is chillingly clear. When Faerie goes to war, not everyone will walk away.