Locke Lemora is the leader of a group of conpersons who call themselves the Gentleman Bastards. They live in Camorr, a city a lot like Venice in the late middle ages, though a lot different, too. We don't know where Camorr is, but there's clearly a backstory, as it seems to rest on the remnants of a much older and far more advanced civilization.
Who the Gentleman Bastards are and how they became the GBs is a tale intertwined with another long adventure in the current volume. Lamora and the other Bastards are engaged in conning a duke out of a good portion of his fortune, but things intefere. Things like the Gray King, who's killing off members of the criminal profession and who's clearly got bad things in mind for Lamora. Just how bad, we discover as we move along.
There's plenty of action, a thick thread of violence running throughout, mystery, and a few surprises, some of them unpleasant. I have the second book in the series, but I'm not sure when (or if) I'll get to it. We'll see.