The Seven Clues — Gerald Verner

Norris Jordan uncovers a fiendish plot and is about to give evidence to the British Government - but on his return to England he is murdered. Having anticipated this, however, he has concealed his evidence within an elaborate series of riddles, each leading to a scrap of vital information; the first going to his friend, Peter Clayton. Aided by the head of the Secret Service, Clayton struggles to solve the riddles - but the duo soon find that their very lives are at stake as the plotters try to silence them...