A swashbuckling pirate getting involved in the machinations of state, as secret agents and spies determine the fate of kingdoms. Of course, in a fantastic setting where the dread pirate Kate rides a dragon -- in exile from the Dragon Brigade -- while nations are floating islands that interact with one another using air ships, and magical beam weapons are alongside cannon.
The setting is effectively fantastic regency era Britain, and has a lot of the conventions of the sort. This works against it to portray the most exciting of action sequences, but provide a comforting structure -- the nobility will be constrained, everybody but the most evil are concerned with honor, and all dialogue is resolutely suitable for children.
One potential complaint that I noticed more about this series than other series in the same world: this was really punctilious about sweeping up and resolving any stray plot threads. There were a few things that felt very unsatisfactorily resolved because of that.
This is definitely a trilogy to read the first book of before diving in ; the books are very consistent so you'll know if you want to read the rest.