THIS has been a surprise best seller, one of those slow burners whose success is due almost entirely to word of mouth, and it's topped the American charts.

It starts in the 1960s when a doctor delivers his own twins. The boy is fine, the daughter has Down's Syndrome. In the panic of the moment and in the spirit of the times, the doctor tells his wife their daughter died and gives the girl to his nursing assistant, telling her to take it to an orphanage in the country.

But she doesn't. Instead she brings the girl up as her own and proves a fierce and protective mother.

It is a powerful tale with two themes: the corrosive effect the dreadful secret has on the doctor's marriage and the nurse's battle to give her adopted daughter an ordinary and fulfilling life.

The dilemmas it raises are as real as they ever were, despite changing times and changing attitudes. It throws out ideas and problems for us all to grapple with, but it is above all a gripping story that eases towards a satisfactory if not conventionally happy ending.