Why Didn't they Ask Evans?. The Agatha Christie Collection. Volume 16

It started when a false step from a misty footpath sent a mysterious stranger plummeting to his death. How was Bobby Jones to know that witnessing the dying man's last words--"Why didn't they ask Evans?"--would send him on a death-defying chase? Bobby and his unlikely partner in detection, Lady Frances Derwent, end up in pursuit of a hauntingly beautiful woman--and a cunning killer who is out to do them in!

This description comes from the 1984 Berkley edition.