AFRICAN LAUGHTER: Four Visits to Zimbabwe by Doris Lessing (HarperCollins). A passionately written memoir of love and betrayal felt toward the "myth-country" that Africa necessarily became for the novelist, who was exiled as an officially "prohibited immigrant" for 25 years for her opposition to the oppressive white government. The country of her childhood is delicately held in memory and measured against the reality of the finally allowed visits during the last 10 years since Zimbabwe was established. Each visit evokes tenuous hope that waxes and wanes with the realities of politics and racism, most poignantly felt during her last visit in 1992, when she acknowledged that while the belated goodwill of the whites is increasing, that of blacks seems to be diminishing. From a letter she received are these words, "When I think of our dreams at independence I want to cry for Zimbabwe. Oh it is so sad, so sad, don't you think so? . . ." And as the Shona people now say, "The sugar is over." And so, sadly, may be the laughter.