It’s still a tentative title, but Book 5: Box 850 is done.
Box 850 is the colloquial term for the British Intelligence Service, specifically MI6, which wasn’t actually acknowledged as existing until 1994.
It seems that MI5 concentrated more on internal threats, and MI6 more on external, but in the case of Mr. Howard, it is not made clear what his connection is to the Secret Service, it is only intimated that he comes from those offices. MI5 has its own colloquial name: Box 500 (after its official wartime address of PO Box 500).
So I suppose I could scoop that as a name instead. What do you think? I don’t know why but Box 850 sounds cooler and more mysterious than Box 500. But maybe that’s just me.