Who can explain just what the magic is that makes Janet Lambert's books unfailingly popular with the young teens -- never a copy on the library shelves! We feel that she is woefully uneven, and that this is not one of her better books, though again it has the gaiety, the glamor, the lively, easy handling of sound social values, the joyfull- validity of a somewhat Pollyanna approach to life. A homey family story, in which the housing shortage moves the Draytons into a madeover barn belonging to mean Cousin Edwin. Christy, a part, gay 14-year old, makes friends with the Bucks County neighbors, round and about New Hope. Happy family incidents, outdoor good fun, young people facing modern problems combine for a readable, surely popular tale. Steady light fare.