This is the third in a "tripytch" of books, after "Bitter Seeds" and "The Coldest War". It's appropriately called a "triptych" not a "trilogy" - a triptych is a picture in three parts, while a trilogy is just three successive books. If you've read to the end of the second book, you might have started to guess why and I will be discussing this in a moment, so if you want to avoid spoilers for the first two books, go away and do something else now (ideally, read those first two books).
If you're still with me, "Necessary Evil" follows Raybould Marsh back from the 1963 of "Coldest war" back to the dark days of 1940. Marsh is a British secret agent and part of the unconventional "Milkweed" organisation, whose job is to conjure up demons to fight the Queen's enemies. But Milkweed has been played, and in '63 the demons break loose to eat the world. Marsh takes a desperate course of action, returning to the Second World War to prevent that. Marsh has also been played by Gretel, one of a secret cadre of German super-soldiers. Gretel can see the future - or rather possible futures - and is trying to engineer one in which the demons fail - and in which she can claim Marsh for her own. As part of this, she murdered his baby daughter (in 1940) by directing a bombing raid at the village to which she was evacuated. that's part of Marsh's motivation for returning to 1940 - not only to save the world, but to save his daughter (and to avoid the subsequent failure of his marriage).
Confused? If not, remember that not only is "old" March now in 1940, but so is the "original" Marsh. And so is the younger Gretel, who can, of course, see forward to the future timeline in which she send Marsh back...
It's a mark of Tregillis's technical skill that he holds these threads and alternate characters together, resolving the story credibly and tying up these loose ends with aplomb. I was also impressed by his characters: they are very human, real people who almost walk off the page - Tregillis sketches their motivations and characters convincingly, and makes them all sympathetic, even horrible Gretel. This isn't run-of-the-mill SF, it is a book very much driven by those real characters, who are not simply there to have the plot happen to them.
The setting was also well described and realised, although slightly less convincing in two respects. First, a few Americanisms slipped through which I think an editor or advance reader could have caught. Secondly, I found the mechanics, especially in the second half of the book, a little unlikely - could you really put on a Naval uniform and wander into the Admiralty in 1940? Was travel as easy as the plot requires? Perhaps, though, it's a bit off to complain about details like that in a book where demons are summed and surgically enhanced soldiers can turn invisible, walk through walls or wield fire as a weapon!
This is a fitting, even triumphant, ending to the "triptych". We have another history of the events told earlier, filling in missing details and completing the picture. So clever is it, it might almost be one of those Steven Moffat "wibbly-wobbly, timey-wimey" Doctor Who adventures.