Promise of Blood blends several different world-tropes together, and in doing so, creates something unique. The largest part of this mixture is a political system similar to that of Napoleonic Europe, with large, feuding nation states, each manoeuvring to displace and subjugate the others. It's nice to see something other than the usual feudal framework that often informs fantasy. Rather than knights, men-at-arms and cheery peasants, we have muskets, artillery pieces, and grimy, aggravated infantrymen. The world, as presented, is politically and socially in flux.
The second part of this mixture is magic, and the sources of magic. The latter becomes clearer as the narrative progresses, but the former is obvious immediately - magic is real. Magic users are powerful, often brutal, and used to getting their own way. In a twin to the social and political upheaval above, the traditional magical power blocs are being upset by the rise of magic users fuelled by gunpowder - a new type of wizard, for a new age. Gandalf and the Hobbits this is not.
Over the course of the narrative, we see a lot of the brutality, grime and worms-eye view that informs a lot of the newer fantasists these days - authors like Joe Abercrombie, Miles Cameron and Mark Lawrence. But there's also a lot of recognisable tropes from older fantastical styles; much like the world created by the narrative, this blend of styles is well done, clever, and easy to read.
The reader is given several narrative strands to keep their attention; one of these is especially interesting, as it presents a murder investigation. Whilst other points of view cover quests and grand political restatements, this investigative, probing style is atypical, and gives a more grounded view of the unique world. Unfortunately, whilst this is a relatively novel viewpoint, it feels a little underdeveloped alongside the more typical views - as the text draws to a (inevitably cliff-hanging) close, the investigation thread rushes into a conclusion - it's still well done, I just wish there had been time to explore it more fully.
Overall, this is sharp, clever fantasy, in a unique and well written world. There are still some rough edges, but the prose is eminently readable - though the reader should be ready for sprays of blood and guts, unflinchingly described alongside gunpowder and revolution. Taken on those terms, this is an excellent book, and I recommend it strongly.