-- The Secret of Chimneys by Agatha Christie.
This book starts off in Africa, when tour guide Anthony Cade gets roped into a simple courier job. Those never end well.
I don't read a lot of Agatha Christie books -- at least, not since a jag in high school -- so I always forget how lively and clever they are. The opening scenes of this one were so witty and modern that I was surprised to learn it is one of her earlier books, published in 1925.
This counts as one of my choices for the Vintage Mystery Challenge, hosted by My Reader's Block.