The "male female detective steampunk duo" is nothing new, but I found this a nice take on it, primarily due to two things: The worldbuilding, and the detective work.
The world is rather different to the usual steampunk (or UF) fare. Instead of a psuedo steampowered advanced Victorian England, this is set in the world of Hy Breasil, which used to be fairyland. It provides an actual explanation why there is magic in the world, why there is a complete mashup of cultures as well as, and perhaps most importantly, both why technology seems stalled at the steam era and why the magic that is left is weak and growing weaker.
In short, people washed up in the fairy world over the centuries, from all over the "real" world--the two main characters names appear to be slavic and japanese--but eventually in such numbers and bearing cold iron that they overwhelmed the natives. Some 200 years ago "The Architect" managed to close the veil between the worlds, preventing further influxes of people, but also preventing them bringing newer technology and science with. While some of the fae interbred with the usurpers, and fae or "ferisher" blood still runs, stronger in some families than others, most people are pretty much normal humans and the Summer and Winter clan are greatly reduced. It is above and beyond the most well thought out premise for a steampunk world I've yet come across.
Secondly, the detective work. One of the main characters has Ferisher blood, but his only power is to read emotions (which he can use both on people and to gather information from crime scenes) and using it gives him blinding headaches and an increasing addiction. Although useful, it's costly, and slightly illicit so while it gives leads it's not proof enough for a court. His partner on the other hand, is the first woman Inspector (i.e. detective) and has had to work her way up from the bottom.
She is very serious about her work, and while she uses every advantage she can get, including her partner's readings, she backs them up with actual detective work: Gathering evidence, following down every lead, being methodical and careful to dot every i and cross every t, knowing that most of her peers are simply waiting for her to fail. While this is still a fantasy novel, it's nice to see some real attention paid to the actual mystery and characters actually trying to solve it rather than flailing around and coincidencing themselves into the answers. Although to be fair, there's a bit of that too, but only a little bit.
Less successful for me, is the romantic aspect. There is no romance to speak of in this book, but there's clearly going to be because I haven't seen this much unresolved sexual tension since David and Maddy at Moonlighting. On the one hand, it's handled quite nicely in a lot of ways. These two have been partners for three years, they've come to depend on each other and understand each other, for good and for ill, and they are fast friends. That a period of intense stress and absolute dependence on each other to simply stay alive puts them both in a position of contemplating what more there could be is actually pretty natural. But for me, it just is a little too much "oh angst" in "... he thought to himself" asides, and given we are seeing alternating POV's here we get it from both sides. My tolerance for overt angst is pretty low (which is why I don't read YA), and personally I found it hammered home a bit hard. Given that every other reviewer on the planet seems to find it a swoonworthy luuuurve for the ages (or they hated the book), it's probably me, not the book.
Overall, it's a solid fun read. Less angst and it would have got a 4 star. And I'll definitely read the next one.