An academic book or journal article is not an isolated, self-contained package of information. Rather, every academic text represents one intersection in a network of ideas and debates that scholars have been tracing through their writing, sometimes over long periods of time. Think of each academic text as one contribution to a scholarly conversation.
In most academic texts, the authors will provide you, the reader, with an outline of the conversation that's been going on so far. This is called the Literature Review. In books, the literature review is often in the introductory chapter. In journal articles, it may also be in the introduction, or there might be a section conveniently labeled "Literature Review."
Whatever it's called, a literature review presents and evaluates previous scholarship on a topic, identifies an unsolved problem or unanswered question in the field, and reveals how the authors plan to resolve this gap in our understanding. In the sciences and social sciences, these elements are often stated explicitly. In the humanities, you may have to infer the gap or the plan to resolve it from the authors' narrative or footnotes.