The Three Coffins #6 by John Dickson Carr

Two murders are committed in such a fashion that the murderer must not only have been invisible - but lighter than air!

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.

'Mr Carr has a sense of the macabre which lifts him high above the average run of detective story writers' - J. B. Priestley