The hollow man

The murderer of Dr Grimauld walked through a locked door, shot his victim and vanished. He killed his second victim in the middle of an empty street, with watchers at each end, yet nobody saw him, and he left no footprints in the snow. And so it is up to the irrepressible, larger-than-life Dr Gideon Fell to solve this most famous and taxing of locked-room mysteries.