The Tainted Cup is an amazing fantasy mystery novel (first in a new series) from Robert Jackson Bennett. For what it's worth, I love love loved The Founders trilogy and quite enjoyed The Divine Cities trilogy so I'm coming into this with some bias.
I've seen this pitched as "Sherlock with kaiju", but I think the Sherlock moniker sells it short for me. The Sherlock / Watson dynamic to me is defined by one where Sherlock is the expert observer, deducer, and dilettante and Watson is the bumbling stand-in for the reader (or at best a medical expert). In The Tainted Cup, I think the sleuthing expertise is split between Kol (the assistant investigator) and Ana (the investigator in charge) and this changes the dynamic entirely in a way that makes the mystery more satisfying structurally. Also, I think personality-wise, they are also quite distinct.
Kol, the point of view assistant investigator, has been biologically modified to have eidectic memory. As he can remember anything he sees, it's his job to be the eyes of the investigator (and also the reader)--viewing crime scenes, talking to witnesses, and poking around to try to dig up as much as he can. Ana, the investigator boss has Sherlockian deductive skill, but whose senses are overwhelmed by the world itself and so stays in her office, relying on Kol to see what's important and bring it back for her to synthesize. There's definitely a power and authority hierarchy here, but what works for me is that Kol's observational competence is strong, and it gives a perfect excuse for the reader to see all of the clues.
I felt like I had put together about half of the mystery myself by the end, but there were still quite a few surprises for me. The closure of the mystery is excellent, and is quite satisfying in the way it brings together every small detail that's been slowly spooled out over time (even, of course, the title). All of this places the book solidly in the mystery genre, rather than just "a fantasy with a murder mystery". It's even complete with a couple of "let me gather everybody to lay out my accusations" style scenes.
But it's the worldbuilding that adds color on top of the solid mystery. The fantasy here is that there this world has experts in biological augmentation, where people are changed to further their station in service of the Empire, often in ways that have negative consequences for them personally. Technologically, things feel like an industrial age that has pursued biological advances rather than gas, such as mushrooms for cooling or worms for lighting. The Empire itself comprises a series of ring walls, where every wet season gigantic titans attack from the sea and must be repelled.
I would have happily read either just a great mystery or a fantasy novel with such an interesting world, but having both together was a delight.
