Look for the author's main arguments. one good way to do this is by reading the title of the book and the introduction section. In the introduction, look out for keywords that relate to the title as well as sections within the book that are accompanied by bullet points or numbers and in bold letters.
Look at the author's research methods. For example, if the book is about the roots of European colonialism in Africa, talk about the sources from which he or she got the research and analyze whether this is consistent with what you've read from other scholars on this topic. If the author used interviews with other scholars, examine if those scholars' statements are credible or not.
Point out strengths and weaknesses. If the author is claiming that Latin America is losing its' Catholic influence because of the rise of Protestant conversions, you can state in the essay that although there is a growing Protestant population there, much of Latin America still has Catholic influence as seen in countries such as Mexico and Brazil.
Mention where further scholarship could be done on the subject. If the book is about the murders of street children in urban Brazil, suggest that more research should be done on how Brazilian economic policies as well as race, class and gender play a role in this violence.