Gosh--I wasn't aware that the books took this turn at all.

I'm not sure it's very well known. Certainly I didn't assume the title meant it literally until I actually read the book. And it interests me because several of the earlier novels are within normal limits of realism for pulp—where your average detective can find themselves tracking down the heirs of a hitherto undistinguished Adriatic principality or confronting a high government conspiracy while under the influence of amnesia—but this one is just straight-up sf and follows through on it to the closing lines; it's not a secret history. If Allingham had lived to keep writing the series, since she had such a documented habit of returning to characters and developments from earlier books and logically catching up with them, I really would have loved to see what she did with the material of this one.

Also interesting to read your thoughts about the order in which to read the series, I believe I tried to power through in publication order and lost steam, I'll have to jump ahead a bit and try again.

Good luck! I hope you enjoy them.