The Labyrinth Index - Charles Stross
This is probably going to be the last novel in his "Laundry Files" series - because after this, how do you raise the 'threat level' any higher?
A being that allows itself to be referred to as Nyarlathotep has become PM of Great Britain, and is behaving like you'd expect an intelligent Elder God would who believes that the continued existence of humanity is key to its own survival.
The US counterpart to The Laundry, the "Deeper State", for lack of a better term, is in league with / in thrall to a being who accepts the name 'Cthulhu'. They have cast some sort of invocation that has caused virtually all Americans to have forgotten that there is such a thing as a President. The President is being hustled around from secret location to safe house by the handful of Secret Service agents who were awake and on duty when that spell was cast.
Mhari, an actual vampire who needs to drink blood to satisfy the parasites that would otherwise eat her brain, has been given a mission by the PM, which she has no choice but to accept. Take a team of assorted other Laundry agents (including an autistic elven sorceress and policeman who has, thanks to the magic that's been unleashed by CASE NIGHTMARE GREEN, become a superhero), infiltrate the US, rescue the President, and break the Deeper State's hold over the country. And, if it's not too much trouble, stop them from trying to wake Cthulhu....
It's a bit tough to keep track of what's going on; the IMF team (no, Stross doesn't call them that, but it does seem a lot like a Mission: Impossible assignment) is broken up into groups for tactical reasons, so the POV jumps around quite a bit. And for followers of the series, there are old characters to remember as well as new ones to get to know. Not much of the humor that characterized the earlier stories, either. But given what's at stake here, I suppose there's not much room for levity.
It's a lot of fast-paced action, and once again Stross seems to have his technical details down pat - at least as far as the needs of the story are concerned. Given how it builds on all the previous novels, you do NOT want this to be the first one of the series that you read. But if you have read them, you'll enjoy this one just as much.