Much more than this is a good book, it's a fascinating relic of early anticommunist sentiment and propaganda. Before Stalin's rise to power, long before the great purge or the cultural revolution, the same tropes were being trotted out that we see today. The material plan of the antagonists here—exposing an embarrassing draft treaty between Britain and Imperial Germany, thereby embarrassing the Tory government and leading to labor calling for a general strike—is unambiguously noble and just. It's only by Christie's explicit declaration that we are to believe these people are evil criminals who will somehow convert a labor action into a vicious dictatorship.

Nor is the plot tremendously compelling even putting aside the political aspect. That one of Hersheimmer or Edgerton must be Mr. Brown is obvious early on. But Hersheimmer has not just seen but had an involved conversation with Brown (impersonating a detective) mere days before and still fails to even slightly recognize that he is also Edgerton. This suggests that either Hersheimmer is an absolute fool, or Brown's powers of innocuousness are fully supernatural.