Although it's hardly the first time "flintlock fantasy", aka fantasy with gunpowder has been done - Paul Kearney's The Monarchies of God in the late '90s for example - Promise of Blood seems to the be one of the first a new wave of books within this sub-genre.
One of the things that intrigued me the most was that the book started in the aftermath of a coup. Normally, this device with be in the middle or end of the plot so this quite refreshing. There's plenty of plots and counter-plots along with a couple of interesting mysteries that hint at what's really going on. The mysteries go hand-in-hand with the world-building, which is revealed slowly, never overwhelming the reader.
The part where this book fails is that by the end, the plot falls back on the standard big battles with lots of magic of many fantasy novels. By the end I was flipping the pages to find out what happened to stage for the next book.
I'm giving this book four stars for a strong first two-thirds, though it's probably closer to three and a half.