The comedian Steve Martin used to end his stand-up show with the line, “We’ve had a great time tonight – considering we’re all going to die some day.” Carlo Gébler has written a haunting book about the final years of his mother, Edna O’Brien. It isn’t a celebrity memoir; it is a book about grief and the anticipation of grief during the decline of illness and ageing. He quotes Susan Sontag and her journey from the kingdom of the well to the kingdom of the sick and all the “hyper anxiety” that comes for those who love the person who is…