The Republic of Thieves: The Gentleman Bastard Sequence, Book Three by Scott Lynch

After their adventures on the high seas, Locke and Jean are brought back to earth with a thump. Jean is mourning the loss of his lover and Locke must live with the fallout of crossing the all-powerful magical assassins, the Bonds Magi.

It is a fallout that will pit both men against Locke's own long-lost love. Sabetha is Locke's childhood sweetheart, the love of Locke's life and now it is time for them to meet again. Employed on different sides of a vicious dispute between factions of the Bonds, Sabetha has just one goal - to destroy Locke for ever.