Equoid

"Equoid" is set shortly before the events of the "The Fuller Memorandum". It's the longest non-novel-length Laundry story so far. And it explains (among other things) precisely what H. P. Lovecraft saw behind the wood-shed when he was 14 that traumatized him for life, the reproductive life-cycle of unicorns, and what really happened on Cold Comfort Farm.