Book excerpt: This book examines the decline of slavery in Northern Nigeria during the first forty years of colonial rule. At the time of the British conquest, the Sokoto Caliphate was one of the largest slave societies in modern history. Rather than emancipate slaves, the colonial state abolished the legal status of slavery, encouraging them to buy their freedom. Many were unable to do so, and slavery was not finally abolished until l936. The authors have written a provocative book, raising doubts over the moral legitimacy of both the Sokoto Caliphate and the colonial state.