The Weight of Evidence — Roger Ormerod

Fred Wallach was not so much a missing person as a disappeared one. It held all the elements of an illusionist's trick.
There he'd been, shut away in bare shed with no way out by window or door -- and yet he had gone. To David Mallin it was not an ideal case for their first as partnership. But George Coe was enthusiastic. A locked room mystery, he claimed. Well...not quite, perhaps, because there was no corpse. Impatient to get home, David quickly solved the mystery, revealing the missing corpse. Unfortunatley he also uncovered another locked room problem, and this time it was one without such an easy solution. The reaching of the end was a harrowing experience for David, who had to face himself before he faced the murderer.