The Killings at Kingfisher Hill Book by Sophie Hannah

Hercule Poirot is traveling by luxury passenger coach from London to the exclusive Kingfisher Hill estate. Richard Devonport has summoned the renowned detective to prove that his fiancée, Helen, is innocent of the murder of his brother, Frank. Poirot will have only days to investigate before Helen is hanged, but there is one strange condition attached: he must conceal his true reason for being there from the rest of the Devonport family.