The author has done particularly well with murder cum mystery among upper- class Americans before. Now, with the Denham family, he has created a group with blood types so blue that when they go to see an Egyptian belly dancer they call her routine a ""stomach dance."" Naturally people so selfconsciously correct and so successfully capitalistic are prime targets for blackmail. And one of them so far forgot himself that he did the blackmailer in on a poor relation's rug. Hard to solve and easy to read.