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How to read your history book:

Find a comfortable, well-lighted place where you won't be interrupted.

Before you start reading a chapter, page through it, looking at...

...the section headings.  Their relative size is a clue to the level of the information included--without the text, they would look like an outline of the main ideas in the chapter.
...the illustrations and maps.  The labels usually will include key terms.
...the "extras"  -- information in boxes or on separate pages.  It often will be supplemental to the main body of the text.  It is a good idea to go back after you finish the chapter and look at these again.  They may be more meaningful after you have read the main content.
...timelines or lists of dates, which put this chapter's information in the larger context
...the first and last sections which will often summarize the big ideas in the chapter.
This process is like checking a road map before you go on a trip.  You know a little bit about where you are going and what you will see along the way.   It reduces your chances of getting lost.

While you read the chapter...

...work on  one section at  a time, stopping to consider what its main idea is before you go on.  If you can't finish a whole chapter as a sitting, do a major section at a time, in order to keep the ideas in the chapter clear/
...avoid excessive underlining.  It's a good idea to mark parts of a section only after you have read it all. To be helpful, marginal notes should be neat  The best, but most time-consuming practice is to take notes. 
...pay attention to geography and its effects on events
...look for personal names; documents, treaties, or works of literature; crucial dates or named eras; named events; technological developments (including agriculture), social, political, religious movements, etc.  but don't become so focused on facts that you lose track of the "big picture."
...try to understand why and how events took place.  This is the important stuff
...remember that history is about change over time.

After you finish..

...reread the first (introductory) section and the last (summary)  section
..page through again to remind yourself of the main ideas in the chapter.
...test yourself by writing a summary of each section after you finish it..
...try explaining what you have learned to someone else.  You can learn by explaining.  
If anything remains unclear, reread until you get it--or ask for help.