PARTING IS ALL WE KNOW OF HEAVEN <i> by Molly Moynahan (Harper & Row: $16.95; 288 pp.) </i>

A smart-alecky novel of contemporary life in New York, “Parting” concerns 29-year-old Cordelia, who can talk about T. S. Eliot in one breath (“April is not the cruelest month,” she concludes, as she tries to get out of bed in February) and “Poltergeist” and “The Twilight Zone” in the next. Her realism is from the pages of women’s magazines: A friend congratulates her on hitting the Big Three-Oh when she’s only turning 29; momentary deep depression ensues. She’s a sucker for beautiful men, which might make her job as receptionist at a health club a tad difficult, but an obvious choice for earning rent while she waits for her big acting break. One of Cordelia’s lovers is a Hungarian musician “with the moral integrity of Fritz the Cat.”

Into this normally neurotic world comes disaster. Her eldest sister, Cynthia, is raped and murdered. Cordelia, the youngest, is thrown into despair. The story becomes a psychological study of bereavement, as Cordelia fights to re-establish her balance. There are intelligent and moving observations about how people come to terms with inexplicable loss. But the pervasive humor in “Parting” is a bit difficult to read: How much tongue is in cheek? The silly choice of names for the sister trio, for instance--Cordelia, Amelia, Cynthia--undermines the observation of a family dealing with tragedy. The author packs in at least a dozen too many of the ills known to young upwardly mobile women, from abortion to eating disorders to rough sex. In addition, it’s difficult to take “Parting” seriously as a study in grief and self-loathing--as it seems to want to be taken--when the story is filled with too much witty New York banter and stories like the following: Cordelia and her best friend pick up young Wall Street types at a trendy club: “They had a pretty dull time except for one hysterical trip to the ladies,’ where they discovered that neither of them knew which banker was named Doug and which was Brad. They solved their dilemma by calling one Boug and the other Drad. The men were not amused, and they finally left for the South Street Seaport in search of more upwardly mobile, well-behaved young women.”