The Man in the Brown Suit by Agatha Christie (Pan, 1959)

Found in a charity shop on a shelf full of Christie paperbacks:

THE mysterious man in the brown suit is a link between a fatal accident at a London Underground station and the body of a strangled woman found at a Member of Parliament's lonely country house.

Enterprising Anne Beddingfeld, back by a newspaper magnate, follows clues leading to South Africa, and there finds herself plunged into a highly dangerous Secret Service adventure.

First published early in Agatha Christie's career in 1924, this book was written while she was still experimenting with different genres.  It is a thriller rather than a classic whodunnit--and her heroine is an enterprising amateur caught in the middle of things rather than a professional detective.  Readers of Christie's Hercule Poirot and Miss Marple series will find this a rather different kind of read!