I think it's the best of the three Devices books. I enjoyed it. It has an exciting story, in large part because it feels shaped by its characters rather than a fabula ex machina as the plot builds up to a revelation of X being Y from legend Z. It's a solid, satisfying conclusion to the trilogy and an easy recommendation to anyone who liked books 1 and 2.
Recap for anyone who came in halfway through... mythical heroes are real. Even if they never lived, the stories have been cycling through people's heads for so long that their archetypes can possess people and make them act like Robin Hood, King Arthur, etc. When this goes wrong, this can drive a country to civil war. (See Book 2.)
The rest of what I've got to say is mostly just commentary, not criticism. The main direction that wasn't taken involves the main character, Jory. I'd have loved to have got inside his head. He's three people: Jory, Robin and Arthur. What's that like? How do you chart the lines of your identity? Do they get on well together? It's a fascinating, unique idea and exploring it could easily have been the book's highlight... but unfortunately it doesn't go there. There are structural reasons for that. Firstly, it would break the plot. Secondly, the narrator is a rather chatty (but oddly anonymous) character in the story, so everything we know is being filtered through his voice and consciousness. This tends to limit the subtlety that can plausibly be put into the characterisation, alas, especially with someone as guarded and unchallengeable as Jory. In fairness my question does eventually get asked and there's a little insight into him right at the end, but that's after nearly 500 pages.
The result of this, alas, is that I don't think Jory's story's conclusion has the emotional weight it should have had. It's fine. It does the job. It's fitting and right. However I think it suffers a bit from Jory having been some bloke whose actions we've merely been watching, rather than feeling.
The plot develops logically from what had gone before. The mythological and political range expands. The only occasional snag with this is that the displayed level of world knowledge can seem a bit weak. p98 talks about "the huge proportion of Crown revenue the High King's gifted in perpetuity to the NHS", for instance. This is... um, nice of him, but let's look at the numbers for 2015/16.
Overall NHS budget: £116.4 billion, which is seven or eight per cent of GDP (the total output of the UK). Crown Estate royal income: £300 million, almost all of which already goes into the government's coffers, with only 15% getting returned to the royal family. That's £45 million, which in government terms is pocket change. It's on the level of headlines like "Labour's false promises leave a £45 million hole in school dinners" or "the government has committed £45 million of banking fines over the next four years to support military charities and other good causes". Considering the NHS's running costs, 24 hours a week, 365 days a year, that would pay for 3 hours 25 minutes. Jory's apparently ensured that the NHS is properly funded, which is great, but that Crown estate contribution is basically gesture politics.
Similarly on p266, "Britain hasn't been trusted with NATO membership since the social upheaval of the War of the Devices." Actually NATO has no rules for expelling members and won't even be kicking out, say, Turkey, despite political witch hunts, torture, extrajudicial killings, persecution of its Kurdish minority and a history of invading its neighbours (Cyprus in 1974).
I'm going into more detail there than the book deserves, though. They're just a couple of lines, easily handwavable with only a modicum of thought. The narrator might not have the full picture. Besides, the trilogy's a fantasy.
None of what I've been saying could be called a problem, though. This is a book that's doing nothing significantly wrong, while telling a solid, thoughtful story that gradually but surely builds up plenty of dramatic momentum. (This even survives more than one chunky timeskip, which could easily have slightly disrupted the flow.) It completes the trilogy satisfyingly, incidentally being a proper trilogy rather than the "marketing trilogy" one often sees, i.e. a success and a couple of hasty sequels. It brings out the big mythological guns that had been being kept in reserve through The Pendragon Protocol
and The Locksley Exploit
. It ends strongly and fixes some minor issues I'd had with earlier instalments. Recommended.