15. The Secret Adversary (1922) — Tommy and Tuppence — 15. The Secret Adversary (1922) — Tommy and Tuppence

Start here when you want velocity rather than a sealed-room puzzle. Christie's second novel introduces Tommy and Tuppence through youthful improvisation and postwar intrigue.

The tone is lighter and more adventurous than the classic country-house cases, which makes it a useful reminder of how wide her range already was.