In general, the introduction and conclusion should clearly present the author(s)'s arguments. Thus, you can get a quick and simple understanding of a book by focusing on these chapters.
The broader concepts and frameworks are more important than the small details.
Look for phrases like: "The goal of this paper is...", "Our goal is...", "We aim to...", "Our purpose is...", and "We propose...". These phrases introduce the overall thrust of a book or paper. They highlight the arguments that the author(s)'s want to make.
Try to summarize the author(s)'s main arguments. Again, look especially to the introduction and conclusion for guidance.
Remeber that no author can address all aspects of a topic. For example, they may only be able to focus on one country or one type of development problem. As a consequence, they will and must overlook important concerns. So, while you should forgive the author for overlooking important aspects, you should also actively identify those limits and explore the implications of those limits for applying the argument elsewhere or for weakening the argument. That is, recognize the limits, but think abou t what those limits mean.