This book comprises a slightly short novel, "The Atrocity Archive", and a novella, "The Concrete Jungle", both previously published. The two episodes connect reasonably well. There is another volume of these stories and I'm not sure how they all fit together, but this volume stands alone well enough.
Stross is a very good world builder, and this book is no exception. The major assumption the world is built on is a very weird one. It posits that the structure of reality is such that simply performing certain kinds of mathematical calculation can actually have meaningful effects which would otherwise be described as magic. Odd as it is, I'd almost be ready to call this science fiction, except that he also includes some other, more traditional magic that doesn't come from the same source and isn't well justified. The mathematical basis of the important magic provides the excuse for enough references to obscure higher mathematics to warm any math geek's heart. The magic is very dark, Lovecraftian stuff; this book could be shelved as horror as well as fantasy. This is juxtaposed against a satirical depiction of bureaucracy run amok; the horror of extradimensional brain eaters is mixed with the horror of accounting for paper clips, so that we're not quite sure if the latter is just comic relief, or if the analogy between the two is really the main point of the book. Against this background, we have plenty of breathless action and things blowing up.
Too far into horror to really be to my taste, but well done for what it is. 7 out of 10.
**** PLOT HIGHLIGHTS -- SPOILERS ****
Our protagonist, Bob, is bored with being a sysadmin in the super-secret occult intelligence bureau, so he's requested to be assigned to active field operations. The book opens with him sitting for hours in the rain waiting for his chance to sneak into an office and collect some contraband data. He ends up successful enough in this operation that he gets assigned to a training course that he could easily have taught himself, but Fred from Accounting, who should never have been in the course at all, violates safety procedures and has his mind eaten by a malevolent being from another universe. Bob kills him and manages to prevent anyone else from dying; as a reward, he is suspended for an inquiry. Then he gets shipped off to San Francisco to meet Mo, a philosopher whose theoretical work is related to the occult, who may want to defect to Britain. (While the US and Britain are otherwise close allies, they do not cooperate on occult matters due to stupid historical politics.) This results in Bob getting a crush on Mo. Then when Mo is kidnapped, Bob breaks all sorts of bureaucratic rules by saving her from being a human sacrifice. Once that gets sorted out, he and Mo are shipped off to Amsterdam, ostensibly to research the occult symbols at the scene of Mo's aborted sacrifice, but really to have Mo as bait to try to bring out the bad guys. The bad guys open a gate into an alternate universe where the Nazis summoned an entity at the end of World War II that ate that universe. They come into the freezing cinder, rescue Mo, and plant an H bomb to destroy the gate. Bob doesn't realize until after the bomb is armed that the explosion will give the evil entity the energy it needs to gate into our universe and eat it too. He figures out how to use his medusa weapon to knock the explosive lenses in the bomb out of alignment so that it won't be able to start a chain reaction, and the world is saved.
In "The Concrete Jungle", the spooks have developed this crazy plan to turn all of the security cameras in the country into medusa weapons as a defense against a major incursion of bad guys that they're seriously worried may be immanent. Unfortunately, a nutcase inside the agency doing the work gets into a conspiracy with one of the bureaucrats who doesn't actually understand that their agency is trying to save the world, but wants to destroy the careers of the real bosses to advance her own career. The metaphorical message for our world, where there is no danger that the all-pervasive security cameras will be turned into actual death rays, but have the very real danger of enabling oppression on a historically unprecedented scale, is rather heavy handed.