Running Close to the Wind by Alexandra Rowland

Avra Helvaçi, former field agent of the Araşti Ministry of Intelligence, has accidentally stolen the single most expensive secret in the world—and the only place to flee with a secret that big is the open sea.

To find a buyer with deep enough pockets, Avra must ask for help from his on-again, off-again ex, the pirate Captain Teveri az-Ḥaffār. They are far from happy to see him, but together, they hatch a plan: take the information to the isolated pirate republic of the Isles of Lost Souls, fence it, profit. The only things in their way? A calculating new Araşti ambassador to the Isles of Lost Souls who’s got his eyes on Avra’s every move; Brother Julian, a beautiful, mysterious new member of the crew with secrets of his own and a frankly inconvenient vow of celibacy; the fact that they’re sailing straight into sea serpent breeding season and almost certain doom.

But if they can find a way to survive and sell the secret on the black market, they’ll all be as wealthy as kings—and, more important, they’ll be legends.