When Wall Street's John Putnam Thatcher and his Down East crony, Henry Morland, started hiking the Appalachian Trail, they hardly expected to stumble over a dead body. Nor did real-estate promoters Eddie Quinlan and Ralph Valenti plan to play hosts to a murderer at their luxurious, new vacation development. Now the head of the Sloan Guaranty Trust Department must again combine his financial skills and unbounded curiosity to solve the murder of a man nobody seemed to dislike enough to kill.