IT IS DIFFICULT not to begin with the murder. In 1796 in Holborn, London, a seamstress at the end of her tether snapped and stabbed her incapacitated mother to death with a carving life, while her senile father wept beside her. Her younger brother was searching for a doctor when the incident occurred. Remarkably, the killer was returned to her brother’s care, and Mary and Charles Lamb remained together until the latter died in 1834. The Lambs have since bobbed through fashions in criticism and biography. Their essays and stories are intriguing cultural indicators, and their sensational past attracts our trauma-hugging present. Kathy Watson’s strongly empathetic biography is the second in twelve months, while the London-loving siblings are central to Peter Ackroyd’s latest novel about the capital.