Is it possible to write a sidesplitting novel about the breakup of the perfect marriage? If the writer is Nora Ephron, the answer is a resounding yes. For in this inspired confection of adultery, revenge, group therapy, and pot roast, the creator of "Sleepless in Seattle" reminds us that comedy depends on anguish as surely as a proper gravy depends on flour and butter.Seven months into her pregnancy, Rachel Samstat discovers that her husband, Mark, is in love with another woman. The fact that the other woman has "a neck as long as an arm and a nose as long as a thumb and you should see her legs" is no consolation. Food sometimes is, though, since Rachel writes cookbooks for a living. And in between trying to win Mark back and loudly wishing him dead, Ephron's irrepressible heroine offers some of her favorite recipes. "Heartburn" is a sinfully delicious novel, as soul-satisfying as mashed potatoes and as airy as a perfect soufflé.