The Secret Adversary

Christie doesn't seem to have the same innate talent for thrillers as she had for detective stories at this point. It plays all its cards face-up, which actually works to forefront the real appeal of the story: the relationship between Tommy and Tuppence.

... Unfortunately they spend much of it separated. Oh well. Still, it's a fine little mystery, and I'm interested in the sequels if only for the knowledge that they are written so far apart that T&T age over them.