Blood and Iron falls very roughly into "Urban Fantasy", mixing magic into modern life. From the world of Faerie, Elaine Andraste is a Seeker for the Mabd, queen of the Fae. Elaine is actually a kidnapped human, bound to serve but occasionally debating revenge while she wanders around doing the Fae Queen's dirty work in the human world. On the modern city side of things, we have Matthew, a human mage from the "Promethean Club", a group dedicated to preventing the bad side of magic getting into our world. Matthew hates the Fae because of something unspecified to do with his brother.
Elaine turns out to be the main character, while Matthew pops up now and again, and it's clear that some big conflict is brewing. Elaine gets tasked with finding and recruiting a Merlin - a mythical archetype that appears every 500 years, along with a Dragon Prince doomed to save his people while also slaughtering and betraying. Matthew also tries to recruit the Merlin, who turns out to be a human woman who isn't at all surprised to find out she has this big mystical secret. Elaine sweeps her off on a mystery tour of Faerie, which is a beautifully rich and lavishly described world. Bear has really gone to town on making the Celtic myths seem fresh and interesting, avoiding cliché even while she mixes Arthurian legends and Dracula into the mix (and that's no mean feat). The problem is that there's loads of stuff going on - there's a dragon in a cave, Morgan le Fey is an enigmatic sorceress in a cottage, Elaine's kidnapped son who she had with a werewolf is the Queen's favourite, there's werewolf politics, the Merlin fancies Elaine, the werewolf fancies Elaine, a shapeshifting Waterhorse fancies Elaine, and so on. There's nothing wrong with a big complicated plot, but Bear seems intent on dropping the reader into the middle of a slice of these lives, then swooping off to the next thing before it's quite developed.
There's some really interesting stuff in here, about the relationship of Faerie to our world, hints at some aged conspiracy to control them, and their relationship to the powers of Hell. The Fae are nicely morally ambiguous, which could be really interesting as you debate whether they should be permitted to survive, but the Prometheus Club turn out to be stereotypical villains near the end so you don't have to worry about which side to root for.
There's no doubt that Elizabeth Bear is a talented writer - you don't rack up multiple Hugo awards for nothing - but her awards are primarily for short form stories, not novels, and on the basis of this I wonder if she is really suited to novels. She seems not to know when to stop throwing ideas, and start developing them, not to mention developing characters.
There's much that is good about Blood and Iron. The writing is excellent and the ideas intriguing. It's also a self-contained, standalone novel, despite being in a series. It's just that you end up not caring enough about the characters to really invest in the story.