You won't find a "right" way to read a history textbook. Here are suggestions that have proved helpful to both college and Advanced Placement Program (AP) students:
Instead of jumping in and tackling your initial assignment, try to get a feel for the book first. Look over the table of contents and glance through the chapters to see how the authors organized the material. You'll notice that each chapter is divided into smaller topical section, which makes outlining easier. Some textbooks begin a chapter with a summary of what is covered, noting important events, or include a timeline. Both are useful study aids. Next, read a chapter or two from different periods. This will give you an idea of the writing style and how the information is presented.
Read each assignment twice. The first time through, your goal is to get an overall view of the time period covered. The second treading is for taking notes. With section titles as a guide, write down key points and significant concepts. You want to pay close attention when authors discuss the causes or consequences of events, summarize the character of a period, point out different interpretations, or make comparisons between one era and another. Study any maps, statistical tables, or other illustrations during the second reading.
The purpose of note taking is to make it easier to review what you've read and to help you retain factual information. You gain nothing if you have 20 pages of notes on a 20-page chapter.
Reread the assignments before a test to reinforce your understanding of the subject matter.
Many college textbooks have accompanying workbooks or student manuals. These contain suggested activities for study purposes, such as true-false questions, fill-ins, identifications, chronological arrangement, and sample essays. If your teacher isn't making assignments out of a workbook, it may be helpful to use one of your own. Look in the textbook section of your local college bookstore; remember that the workbook you find may not be written for the book you're reading for your AP class.