For anyone who has ever been married, divorced, remarried and a stepmother. Or who needs a laugh. Ephron's third work is less a chronicle of her own life as a stepmother than it is a pastiche of vignettes, one-act plays, transcripts of telephone conversations, diary entries and musings, e.g., on what it means to be a stepmother when the family goes out to dinner. (It means being the one the children--his children--refuse to sit near. ""I am the green lollipop. (Larry is the red one.) In other words, in the battle between the siblings, whoever gets me loses."") It is Ephron's contention that today's family dramas need some comic relief: his and her versions of the divorces of Mickey and Minnie, Dick and Jane, Olive Oyl and Popeye. And she explains why the movie Stella Dallas is really about stepmothering. Several chapters, however, seem out of place simply because they have little, if anything, to do with the yours, mine and ourness of mix 'n match families. Pieces about her high-school reunion, a repairman, Santa Claus and a maybe, but probably not, priceless work of art are fine and funny but they leave you wondering what they're doing here. Still, this is a small quibble when you consider that this is the best medicine for those who need a laugh about 4 a.m.--when they're wide awake with anxiety about the family they've taken for better or worse.