Textbooks are a fantastic resource. Usually in undergrad though, they are pretty inefficient b/c they don't teach a course, but they teach an entire topic. As I've continued through my education, I found flipping back through my texts as the most effective review of material.
My advice for reading them is pretty simple. Underline key thoughts and phrases under each specific heading in the book. Then at the end of that heading, read just the underlined phrases. You should be able to remember everything about the topic. Continue doing this throughout the chapter. At the end of the chapter read each of your underlined sentences again. If you can you recall each topic, you're all set. Not only have you internalized the important points of each heading, you've linked them mentally to the more minute details, and you've cut your work down in re-reading the chapter by probably 90%.
Edit- This method works w/ scientific journal articles for the most part as well.