And Then There Were 37: N or M? by Agatha Christie

Rejoice or beware, per your preference, because Tommy &
Tuppence, those alliterative lovebirds, are back! And in wartime no
less: the spring of 1940, to be exact (and Christie is very exact).
Join us as we discuss this mid-career thriller of Christie's, which
is in many ways quite typical (poor Tommy and his poor head!), but
in others quite unusual....