The Dutch Blue Error - William G. Tapply

Boston lawyer Brady Coyne investigates a philatelist fatality in "a first-rate mystery . . . a knockout climax, charged with irony" (The Washington Post Book World).
It is a small paper square with uneven edges, dark blue in color and bearing a smudged portrait of a long-dead king. It doesn't look like much to Brady Coyne, but the stamp known as the Dutch Blue Error is one of a kind—a philatelic freak worth at least one million dollars. It is the prize possession of Ollie Weston, a wheelchair-bound Boston banker, and it is valuable enough that for its sake, several good men will die.
A fellow collector contacts Weston, claiming to have found a second copy of the Error—a claim that, if truthful, would destroy the stamp's value. Weston sends his attorney, kindhearted Boston lawyer Brady Coyne, to purchase the rogue stamp for two hundred fifty thousand dollars, but just before the hand-off, the collector is killed and the stamp disappears.
Find the stamp and Brady will find the killer—but that will involve risking another one-of-a-kind item: his life.