Something we didn't cover, being unaware of it at the time:
World War Cthulhu: The Darket Hour. Cubicle 7, 2013, $39.99.
This is a setting book for Call of Cthulhu during WWII. Its core idea is "Network N", a British secret organisation that recruits from MI5, MI6 and SOE staff, telling them about the Mythos and giving them additional investigation missions to be done during their missions for their main employer. This seems to be a good way of creating conflict, while leaving the characters unable to explain what's really going on.
The upside is that the mysterious N has access to a lot of Mythos information and experimental psychotherapy that reduces the SAN impact of regular encounters with the Mythos. But you're still doomed, of course.
The book's wider usefulness comes from its clear accounts of how intelligence operations work, and the differences between them and paramilitary work. It also has broad, but thin, coverage of British intelligence operations throughout the world during WWII.