First off this book is a big step in the roleplaying game, bringing it in line with the latest novel in Charles Stross' Laundry Files series, the Apocalypse Codex. As a result it gives you details of the events of the novel as well as an insight into External Affairs and the series' bete noir among the various occult intelligence agencies the Black Chamber in addition to some other goodies. The write up for External Affairs gives an overview of this part of the Laundry, going into a great deal of detail about their status and operations and how to best use them in your campaigns.
The chapter on the Black Chamber gives them a detailed history and organisational insight into how they work. Worryingly there are also details for running Black Chamber campaigns, presumably for players who want to be able to be complete arseholes (as most of the Black Chamber do seem to be that way inclined). One thing I'd be very interested in is seeing how official the information presented here is. I'm guessing Stross and Cubicle 7 will have been very close and careful in developing it so there's no danger of future novels contradicting what's here, but stranger things have happened.
The meat and drink of the book, for me at least, is the chapter on the sleeper in the pyramid, a figure that's become very important in the latest Laundry novels, acting as a Lovecraftian variation on the Four Horsemen of the Apocalypse, something that needs to be woken in order for the stars to be right and CASE NIGHTMARE GREEN to come to pass. This section of the book is well written and very useful for GMs.
The idea of playing the Laundry in other times than the present was introduced in the Agents Handbook and God Game Black pushes this further, actually detailing the setting before the Laundry came into existence. This makes an extant tie to the rest of the Call of Cthulhu game books (including Cthulhu Britannica). This seems to be a smart move by Cubicle 7 but I'm a little ambivalent about it. For one thing, much of the joy of the Laundry lies in the weird mixture of espionage, the Cthulhu Mythos and computer based shenanigans, which earlier periods simply don't have. On the other hand it seems foolish to ignore the wealth of history available to Cthulhu players or to take advantage of the many campaigns that are out there (how long before someone takes the proto-Laundry and uses them in Masks of Nyarlethotep or Horror on the Orient Express I wonder).
Interestingly we get updated profiles of the series' protagonists Bob and Mo' (even if she is presented as Agent CANDID), as well as a score more, mostly relating to characters from the Apocalypse Codex and the Fuller Memorandum. In many ways these feel like the least useful part of the book, but at the same time they do allow you to include the likes of Perephone Hazard in your campaign, which can be no bad thing.
Finally then to the adventures at the end. I must admit to only skimming these, as I tend to only read adventures in full when I'm going to run them. They seem good and tie nicely in with the escalation theme running through the book.
All in all this is a good addition to the line, one that moves it on apace. There are some typos and grammatical errors, but that's inevitable in any roleplaying book. Sadly it's not a book that I'll be using any time soon, as the Laundry game I run is nowhere near the point where I'd use the information in it. This is definitely not a book for beginning players or GMs.