Most people believe that textbooks are to be read by starting at the beginning, and working through in a linear fashion to the end. This is not necessarily true. Textbooks generally have a large amount of detail, but they are organized around central themes. If you are unclear about the theme, you will not be able to make sense out of the detail. Therefore, you have to start by figuring out the theme:
1. In each chapter, look for the main theme. This might be located at the beginning of the chapter, but it might be at the end (in the conclusion or summary). It might be in the introduction to a section, or even in the introduction to the whole book. Write down the main theme of the chapter.
2. Identify the sections in the chapter, and determine what they have to do with the main theme. Write that down as well.
3. Read the sections, taking note of significant names of people and places, events, words, and concepts as you go. Define those which you don't understand. If necessary, look up the terms in a decent encyclopedia (e.g., the Britannica).
4. Ask yourself - what story is being told in this text? Are there things (people, events, perspectives) being left out?
5. Write a short summary for yourself of the chapter ("This chapter is about..."). Include the significant details you noticed in step 3.