The Doorbell Rang by Rex Stout

Nero Wolfe is sleuthing as usual in these three mysteries. In the Best Families deals with Mrs. Rackam, an aging millionaire who approaches Wolfe to investigate why her young and penniless husband suddenly and mysteriously has large sums of money. Wolfe's inquiry leads him to a confrontation with Arnold Zeck; later a letter bomb causes Wolfe to resign from detective work and go into hiding, leaving his assistant, Archie Goodwin, to solve the case.