Among Thieves very well could rank as by far the highest on the "book that caught me most off-guard" list. I went in with lukewarm expectations; the ratings on Goodreads were good, but not consistently exceptional. Reviews tended to be positive, but they certainly weren't universal, and I hadn't heard much scuttlebutt or discussion about this book in any of my usual venues (r/fantasy, book blogs, Goodreads). Part of that could be due to the book being released a couple years ago, but I did not have any data either way.
This novel is about a thief/rogue/spy type character, Drothe, who lives and involves himself with the criminal underworld of the Kin. Along with his buddy Bronze Degan, he ends up pulled in several directions, due to his affiliation with multiple crime bosses, some run ins with powerful lords in the Grey Princes, and his alliances and oaths to protect or assist multiple people. The story is fast-paced and interesting, and involves multiple changes of direction and boatloads of high-intensity battles, both of wit and of steel.
The writing is tight, the prose approachable and enjoyable, and the dialogue is entertaining and fun, reminiscent of Scott Lynch at times. The characters tend to be fairly three-dimensional, and struggle with the consequences of their actions and decisions. The fairly frail thief type + bodyguard dynamic isn't exactly new ground or remotely original, but the execution here is superb, the relationship interesting and complex. The world is well defined, the concepts of the Degans was unique and dramatic, the people and places all well fleshed out, the history of the world done enough to give a feeling of reality.
The ending left a bit to be desired, but not enough to detract from the book for me. I loved this book and didn't want it to end, and I'd love more from this author for sure.