The world of The Cinder Spires takes place some two miles above the wild and dangerous surface of the Earth. Most of humanity lives in massive structures called Spires, and these enclosed ecosystems seem to run themselves much like individual countries. While most of the populous never leaves the Spires themselves (and apparently there is the risk of madness for going outside without protection), trade, battle and other interspire interactions take place on airships of various sizes, shapes and armament.
One such vessel is Predator, a merchant ship owned and run by Captain Grimm, a former rising star in Spire Albion's military who was dishonorably discharged due to cowardice, though it quickly becomes apparent that there is far more to that story than is let on, and even by the end of the book, not many details on that part of Grimm's history have been revealed. What is clear, especially when Grimm is observed by some of the other main characters from the book, is that the captain is no coward.
While Predator and her crew are a main focus of the book, there is a whole party of other characters that are introduced and eventually thrown together in order to investigate a threat to Spire Albion, the primary location of The Aeronaut's Windlass. Most of the other main characters are new recruits to the Spirearch's Guard, the protectors of the Spire's monarch. These include Gwendolyn Lancaster, heir to one of the most important houses on Spire Albion, Bridget Tagwynn, the heir to a very minor house that once held a lot of prestige and Gwendolyn's cousin, Benedict, a warriorborn (think human with a lot of cat-like features), who has already served several tours in the Guard and has apparently already caught the Spirearch's attention.
Accompanying Bridget is a rather unusual character, Rowl, a cat. Not a cat-person, or some kind of cat-like creature. A basic house cat, though to call him that would be rather offensive. It seems that in the world of The Cinder Spires, at some point, people learned to communicate with cats, and what was an informal arrangement between cats and their humans became a much stronger agreement. While most people don't have the linguistic knack to communicate to cats in their native tongue, Bridget does, and when she goes off to the academy, Rowl is sent to join as well, not only to protect Bridget, but to be a representative of cats and formally make their house known to the Spirearch.
While everything appears to be going well for the new recruits at first, chaos breaks out when another Spire attacks and invades Albion. In the heat of battle, Gwendolyn, Bridget, Rowl and Benedict prove themselves to be a solid team, and because of that, they are sent on a special mission.
The world and magic of The Cinder Spires is driven by a force known as ether. The energy flows around most objects, but some items can take them in, or be affected by them. The crystals used to levitate ships is powered by ether, the sails used on those same ships use the ether currents like winds, and even the weapons of The Cinder Spires use specialized ether crystals, but there are also people who are affected by ether. These etherealists are the magic users of the world, and apparently being able to use ether can drive you mad, at least to some degree. The final two protagonists of the book are Efferus Effrenus Ferus and his apprentice, Folly, and they are the reason for the mission that the recruits are sent on.
It seems that Efferus and Folly have stumbled on some kind of information linking the attack on Albion to a greater threat. When Efferus approaches the Spirearch and convinces the ruler that he needs to investigate in one of the lower levels of the spire, the Spirearch calls upon Gwendolyn, Bridget, Benedict and Rowl to act as Efferus' guards and helpers. Their transportation to the other part of Spire Albion? None other than Captain Grimm, of course.
While The Aeronaut's Windlass is an overall good book, it's main flaw is how long it takes to really get going. With the exception of some short stints of action early in the book, the novel didn't really grab me until everyone was all in one place and the adventure seemed to actually get started, and there was a lot of build-up before that happened. I attribute most of that to this being the first book in the series and that a lot of the setting had to be established before the characters could really get going on their journey, but it does make it a tough book to get started. Thankfully, that does go away, it just takes a solid third of the book before that happens.
While it didn't grab me immediately, the world building that Butcher puts into The Cinder Spires is top notch. How the strange sky-based world works, as well as the more technical aspects of the airships and weapons are all detailed well, and even when a few twists about the apparent nature of magic are hinted at, it doesn't feel out of place in the already established mythos.
Similarly, one of the reasons the book takes a while to really get into the main adventure is because a lot of effort is put in early on to really flesh out each of the characters. Before everyone meets on Predator, you know who all of the characters are and what their nature is. I especially enjoyed how Butcher portrays cats and Rowl specifically. I felt like every segment that took his perspective really stole the scene, and any cat owner, or should I say, cat-cohabitant, can easily recognize Rowl's attitude.
The Aeronaut's Windlass takes some work to get into, but once you're hooked, the book is a fast ride to the end and you are left wanting to see where the story will go next. Hopefully, the next installment of The Cinder Spires won't have to spend quite as much time on world building and it will be able to grab readers from the start.