"My Side" is an encore to Miss Gordon's highly successful "Myself Among Others." At 80 she is still writing chattily, with diaries and letters as a prompter. The most moving parts, which thread the entire narrative, are her remembrances of Mama who died when Ruth was first trying to make Mama proud of her. Her marriage to Gregory (who died young too) and the break-up of her second marriage to Jed Harris are more summarily skimmed over, but then Ruth is a life-oriented pragmatist. She's also a cheerful raconteuse primarily of the plays and films which brought her prominence, beginning with her rehearsal for a church pageant on the chamber pot; she went on to play Nibs with Maude Adams in Peter Pan, got her first real role in Saturday's Children, and reached her apogee in Ethan Frome. But she's never been far out of sight in recent years. Neither are her lifelong friends--Woollcott, Helen Hayes, Thornton Wilder, as well as third husband Garson Kanin. She came a long way from Wollaston, Massachusetts; she still sleeps with a pink teddy bear; and she's undimmed by all this crowded experience, good or bad, and ready to get on with the next.