Black Orchids by Rex Stout

A remarkably rare black orchid at a flower show lures Nero Wolfe from his comfortable brownstone. But before the detective and his sidekick, Archie Goodwin, can stop and smell the roses, a diabolically daring murder puts a blight on the proceedings. The murderer to be weeded out is definitely not a garden-variety killer. Wolfe must also throw his considerable weight into another thorny case, this one involving a rich society widow bedeviled by poison-pen letters -- and a poisonous plot as black as Wolfe's orchids with roots even more twisted.