The physical and emotional complexities of childhood leukemia, in the composite case of four-year-old ""Ellie"": a patient at Memorial Sloan-Kettering (in N.Y.C.) and Johns Hopkins, two of the pioneer centers of leukemia treatment. As Tucker's Ellie is first diagnosed, we learn that the picture for children with the most common type of childhood leukemia is brighter than it was even ten years ago: major advances in chemotherapy and radiation therapy have vastly improved chances of survival. Another important new technique is bone marrow transplant. A sibling's marrow (in Ellie's case, that of her eight-year-old sister) may be used to replace the leukemic child's disease-producing marrow, largely killed off by chemotherapy and radiation. Other aspects of this difficult battle well depicted here are the emotional and physical effects on the patient (how does it look to a four-year-old?) and the emotional upheaval experienced by parents (marriages totter), by siblings (if the transplant fails, donor siblings may suffer extreme guilt), by friends and neighbors, even by doctors and nurses (it's impossible not to get involved). Tucker, a medical writer and former editor of Scientific American, covers so much, indeed, that his case study comes to seem somewhat contrived, which ultimately keeps readers at a distance from Ellie and her plight. But neither this weakness nor Tucker's persistent, artificial use of the past tense (Johns Hopkins ""was located in a poor section of the city"") prevents Ellie's story from being a helpful, realistically hopeful report for parents in need.