I have to confess to having been skeptical of the premise of these books when I first heard of them. Boy howdy was I wrong. Stross pulls it off beautifully, this cock-eyed intersection of spy thrillers, IT wankery, civil service drudgery, and eldritch horrors from beyond the boundaries of time and space. These are highly entertaining books, and by the third volume, he's developed a definite series arc pointing ahead. The fourth volume sustains that arc, and with the its ending lands Bob Howard, your humble narrator, in some seriously uncharted waters that I can't wait to explore in the next volume or two.
There's a very strange charm to this series, which I suspect evolves from the unlikely premise as explicated by the goofy insouciance of narrator and protagonist Bob Howard. (Though in truth Angleton might just be my favorite character.) They're certainly structured and written like spy thrillers or adventure novels, but the sensibility is so very much from the darker corners of fantasy, not to mention outright horror fiction. More to the point, entertaining as hell.
Highly recommended, even if dark stuff isn't normally your bag.