Seasons of Death — M.K. Wren

Book five in the Pacific Northwest mystery series featuring bookstore owner and P.I. Conan Flagg.
Forty years after Leland Langtry ran off with his secretary and $10,000 in company funds, his body was found inside a boarded-up silver mine tunnel. His ex-partner's knife is still in his chest, and although dead, is declared guilty. But the convicted man's widow doesn't see it that way, and hires private investigator Conan Flagg to find the real murderer--an impossible job made tougher by the curious reticence of nearly everyone in town whenever Leland Langtry is mentioned....
"Conan Flagg may join that intrepid club of private eyes whose members include Ellery Queen, Nero Wolfe, Hercule Poirot, and Sherlock Holmes."
THE SEATTLES TIMES