The Christmas Mystery ($23.00; Nov. 1996; 256 pp.; 0-374- 12329-2): Intelligent Nativity story by Norwegian philosopher/novelist Gaarder (The Solitaire Mystery, p. 621, etc.), full of heart and warmth. As ever, Gaarder offers a didactic subtext. Joachim's father buys him a homemade, 40-year-old Advent calendar from a dusty bookstore. The calendar has a door to open for each December day leading up to Christ's birth. Behind each door lies a picture. But when Joachim first opens the calendar, a small slip of paper falls out containing a substory that goes on day by day as each door is opened and a fresh sheet appears. The substory tells of little Elisabet, who runs off in pursuit of a lamb only to find herself on a pilgrimage to Bethlehem. Each day's episode adds a new pilgrim to her group, until there are wisemen, angels, shepherds, and sheep; two millennia of history unfold backward. In yet another story, a girl named Elisabet, who was kidnapped 40 years ago, turns up as a middle-aged woman on Christmas Eve, mingling the celebration of Christ's birth with a contemporary event. For adult children.