Substitute Prisoner (audiobook)

When Herbert Whitmore learns that the husband of the woman he loves has threatened to kill him, he disappears for six weeks, then returns to his office and is killed by a gunshot wound. The only problem with a suicide verdict is that the only gun in Whitmore's office has not been fired. The only problems with a murder verdict are that no one other than Whitmore entered or left the office, no one hid there, and the solid walls made shooting from a distance impossible. The detective figures out the "how" in short order. But the "who" is another question altogether. - Summary by Tom Penn