X-link to the DRM-free thread for an official RPG tie-in via Bundle of Holding:
Bundle of Laundry contains a repeat offer for the licensed adaptation of Charles Stross' excellent and entertaining The Laundry Files Lovecraftian spy thriller spoof urban fantasy series (portions of which are available to
read for free as Hugo-nominated novelettes over at Tor.com, and via
links on the author's own website), and is a very entertaining worldbuilding-exploring supplement to the main series, even if you don't game. (And it handily compiles a lot of the known info about the world into a nice near-encyclopedic format for read-along reference.)
I bought this last year when it was on offer coinciding with the then-upcoming release of the 5th book, and can recommend picking it up to go with the new release of the 6th book,
The Nightmare Stacks, out today, if you like these sorts of things.
NB: although this is the official adaptation, the additional info is not canon for the series, just spinning off from the books and a few possibly-Jossed notes given to them by Stross, in case you were hoping for series-relevant tidbits.