The first volume of the series dealt with werewolves and the second book with vampires. Iron Kissed is the book which expands on fairies in the Mercyverse. I'm not usually a fan of the Fair Folk in stories because very few manage to really give them a unique personality versus a faux Medieval aesthetics. Iron Kissed avoids this problem by making the Fae a race which is at war with itself. There's the fae who want to be among humans, the fae who hate humans, the fae who are almost feral, and those who are powerful like gods.
The premise is Mercy has a debt to pay to the Fair Folk, which is something you never want to be in a position of. Her former boss at the autoshop, Zee, wants her to investigate a murder which has been committed on the nearby fae reservation. Contrary to all the laws of narrative logic, Mercy Thompson solves the murder in a couple of chapters. Unfortunately, no sooner than she does then the perpetrator turns up dead. This is a problem because the victim was a federal government employee and Zee is soon arrested for the murder he (probably) didn't commit.
I really liked this novel because it got into the nitty gritty of fae politics. We've got a society of individual "monsters" for a better word who have a variety of powers and needs but who all somehow get along. The existence of the Gray Lords as the godlike fairies who dominate them is a good bit of world-building. I also like the fact we got into how regular humans react to them. My favorite part of the book is when Mercy Thompson has to get a lawyer for Zee and finds the only one who wants the job is someone who has severe issues with the fae herself.
Among mortals, we find out there's anti-fae hate groups, awareness groups, and organizations designed to find out more about them. The fact Mercy comments, unlike real life minorities, the Fae ARE dangerous also puts this series above things like True Blood where we're axiomatically supposed to side with the supernaturals. Sadly, we're in a present environment where the idea of hate groups being less than incredibly dangerous is painfully naive.
I like this is a story which is primarily solved by Mercy using her brain versus violence or deus ex machina. Finding the actual killer when there's potentially an entire reservation full of them is a good challenge for our favorite werecoyote. We also get some good progress on the Adam, Samuel, and Mercy love triangle. I appreciate Patricia Briggs didn't try to drag out the relationship drama longer than it was necessary.
While not spoiling the book, I have to say I really appreciated the villain in this book. Before the final section of the book (see below), I had to say it was a nice change from the usual vampires and werewolves of the story. It goes to show evil comes in a variety of shapes and sizes. I also liked the development for Zee and Mercy's relationship.
Iron Kissed is my favorite of the Mercy Thompson novels and I would normally give it a 5 out of 5 but I feel the need to mention there is a sexual assault at the end of the book. This is something which will undoubtedly bother some readers despite it being handled tastefully. Do I recommend Iron Kissed? Oh yes, certainly, as it's probably the best "mystery" of the series. The last section of the book, however, deals with a very traumatizing subject and I worry it was unnecessary for an otherwise light-hearted series. Obviously, it's up to individual readers to decide.