The occult thriller is an easy form, if you're a lazy writer unwilling to think through the logic of your plot. Doing it well is hard: managing the interaction of supernatural and real worlds, while allowing your characters to work in both is difficult. Devereux does it very well indeed.
His authorial tools are deceptively simple. A calm, deadpan narrative style, echoing Tim Dorsey and Louise Welsh. A central character who accepts that he's unpleasant - a killer in the service of his country - but knows that that's required by the job. And the basic but unfashionable method of sparse, concise writing, getting his story into no more pages than it needs. Some publishers would have insisted on this book being "filled out" or "backstory added" to 500-plus pages; it is greatly to Gollancz's credit that they allowed Devereux to tell it in 272.
Not that it feels that long.
Devereux has a truly remarkable control of narrative pace; the last time I read a first novel with such superb control of the pace of events was The Hunt for Red October. Action, reaction, deduction, recovery - all this is handled in less space than it takes to write about it. We are seeing a real talent emerging here. Keep an eye on him.