Point out strengths and weaknesses of the book. This allows you to go beyond giving a summary of what the book was about. For instance, if the author claimed that affirmative action policies are very beneficial to minorities, you could disagree. State that according to certain scholars, affirmative action policies may hinder minorities' success. Their employers and co-workers may assume they have the jobs because of affirmative action and not due to having the right qualifications.
Mention other research that has been done on the subject. If the book deals primarily with religious persecution of Christians in contemporary Africa, you could point out that other scholars have done similar recent work on this topic. You might add that further research should be done on religious persecution of Christians in other parts of the world, perhaps Latin America and certain parts of Europe.
Talk about how this book contributes to a specific field in general. For example, if it is about inadequate educational resources in inner-city schools, mention the ways in which the book contributes to the field of sociology. You could say that the book covers the role of neighborhood grassroots organizations as a potential source of improvement in inner-city schools, and how some scholars have neglected their importance in rebuilding American education.