The Hollow Man by John Dickson Carr (Penguin 0-14-008306-5, 6th printing, n.d. Cover by Jonathan Gibbs)
Two murders are committed in such a fashion that the murderer must not only have been invisible but lighter than air!A classic locked-room murder mystery, named by Edward D. Hoch as his favourite impossible crime novel. Carr was one of the masters of this kind of novel, wherein the murder appears to have been performed by some supernatural means until the detective finally unravels the solution. This particular novel is famous as its detective, Gideon Fell, offers a lengthy lecture on the general mechanics and development of the "hermetically sealed chamber" in which Carr knowingly has Dr Fell admit: "We're in a detective story, and we won't fool the reader by pretending we're not."
__According to the evidence, this person killed his first victim and literally vanished, and then struck again in the middle of an empty street - with watchers at either end who saw nothing and no footprints in the snow.
__It was a problem that Dr. Gideon Fell - huge, rumpled, with flowing cloak, eyeglasses and cane - regarded as one of devilish illusion and impersonation.
An invitation to the country home of Baron Arnheim Wulfran, ambassador of a hostile foreign power, was the beginning of a dark journey for Ruth. Hoping to wrest from the infatuated Baron the secret of the whereabouts of Dallas Kerr of the Secret Service, she was plunged into an adventure fraught with terror.Horler is almost forgotten nowadays, although he was hugely popular in his time. A workmanlike writer, he was the creator of Tiger Standish and various other series characters.