Getting Happy with Parker Pyne: Death on the Nile by Agatha Christie

That's right; do not adjust your screens; before she wrote her much
more famous Death on the Nile, Christie wrote a short story
featuring her "heart specialist" Parker Pyne on a trip up that same
(in)famous river.... Join us as we plumb the depths of this neat
little puzzle mystery, which features a glancing reference to a
substance these two American readers found most intriguing of all:
Bovril.