Of course, you will read your book, but just reading is not enough. When you read a chapter, look for the main ideas and supporting facts. You can find clues to the main ideas in the chapter by reading the section titles. Look at them and notice that they occur in a certain order. That order is a clue to what is important.
Some books have main section titles and subtitles within a section.
Look through a chapter before you read it, and note the headings, read the figure captions, skim the tables. Get a preview of the chapter and its main ideas before you read it in detail. You can also recognize main ideas by looking for a topic sentence in each major paragraph. The topic sentence is the one that tells what the paragraph is about. It is usually, but not always, the first sentence in the paragraph.