Seaborn (The Seaborn Cycle #1) by Michael Livingston

The seven houses of the matriarchal Seaborn have plied the seas of the Fair Isles for centuries, trading among the islands and fending off the attacks by the fearsome Bone Pirate.

But suddenly, out of the night sky, a common enemy appears – the Windborn, who come without warning to raid, burn, and kill.

Hoping to turn the tide, Shae - the Bone Pirate's first mate - enacts a daring plan to fight her way aboard a Windborn vessel. The raid yields a prize - the airship's captain who is, to Shae's shock, a man.

Together with a reluctant heroine, Bela, they learn the truth of their shared history: the Windborn and Seaborn come from the same people, split apart by blood magick when a race of immortal mechanical men betrayed their human makers.

Now, these unlikely allies must make a desperate journey to confront the secrets of the past – and stop the dark magick at its source.