Drawing on his professional experiences, Murphy (a pen name) initiates with this mystery a series involving legal practices on Wall Street. Reuben Frost is 74, former chief of the eminent law firm of Chase & Ward; with his wife Cynthia, a retired ballerina, he helps Luis Bautista of New York City's detective bureau to solve a knotty case. The staff of the firm is shaken when partner Graham Donovan dies at lunch and Reuben finds poison in the victim's carafe at the office. Murder from a slow-acting drug is the verdict and the suspects are many: Graham's son who hates him; a colleague whose wife had been the dead man's lover; the secretary known for her years of devotion to Graham. By the time the Frosts and Bautista bring the felon to book, readers will wish the author had emulated Reuben's ''complete if judiciously edited account'' of skulduggery.