Maths is magic and the Elder Gods are real. Bob Howard works for the Laundry, the UK's secret intelligence agency that protects you from these facts and their consequences, which include such delights as insanity, possession and having one's soul eaten.
I'm reading this series in random order, having gone 3-1-2-4. This is book 2, in which our reluctant hero has to stop the megalomaniac business man turned would-be Ruler of the World by circumventing a fiendish geas that will only allow a highly unrealistic type of super-agent to get close to the villain. Yes, some-one has to become
James Bond in order to save the world! Cue stereo-type and cliche subverting jokes such as a glamourous hottie that's really a succubus and minions that are literally zombies.