I must start with a warning: this book is definitely not for everyone. Also, be aware that it contains two stories, the second one being a novelette, The Concrete Jungle (which I might have liked slightly more than the novel itself, perhaps because by then I had a stronger grasp on the world developed here, and could therefore understand it better?).
If you're not at least slightly familiar with H. P. Lovecraft's mythos; if computer science is not your cup of tea, and technobabble tends to irritate you; if you have no previous knowledge of UK intelligence services; if "Ahnenerbe", "Thule-Gesellschaft", or "Church-Turing thesis" don't mean anything to you; then you probably won't like The Atrocity Archives.
On the other hand, if those aren't a bother; if you like spy stories full of jabs at bureaucracy and red tape; if you're looking for dark humour; and if the idea of mathematics applied to magic seriously rocks your boat... then go ahead.
My only real problem with this book was the portrayal of female characters: either the Damsel in Distress or the Crazy Bitch, which didn't make up for much diversity, hence my 4 stars and not 5. The rest I loved.