I'm sure this comparison is being tossed out all over the place, so I'll get it over with. This feels a lot like Scott Lynch's books, just not as grim. It's Scott Lynch lite, if you will.
It's not derivative, though. Among Thieves feels like the Gentleman Bastards books because of its subject matter, not its plot or characters (sort of like all heist movies resemble each other superficially). Main character Drothe is a Nose--someone who collects information--for a crime boss in the city of Ildrecca. He also runs a tidy side business dealing in relics.
Things are turning bad in the part of Ildrecca known as Ten Ways, where none of the city's bosses have the upper hand. Drothe's boss wants him to find out what's really going on. Drothe never wants to go back to Ten Ways--he's just happy he got out in the first place years ago. But when a relic Drothe has been after turns up somewhere it shouldn't be, and assassins turn up in his bedroom, he realizes there's a lot more going on than anyone suspects.
The plot is intricate, fast-paced, and clever. There's plenty of violence, but Drothe is a nice guy with a strong (if skewed) sense of morality--likable enough that I rooted for him without hesitation. The worldbuilding is solid too, with lots of unique details about the empire's history. In fact, there's an awful lot of what felt like infodumping toward the beginning of the book, although it's not as bad as the book goes on. While the story doesn't flag, all the extra information makes the book longer than it probably needs to be.
I enjoyed Among Thieves a lot despite its length (and it's not all that long, really; I'm just an impatient reader). It stands alone, but it's also definitely set up for a sequel. That's fine with me. I'll be reading it.