Reading is a lot like listening. When you're truly interested and engaged in what someone is telling you, you can listen and absorb what the other person is saying — even if that person is speaking to you at a fast clip. However, if you're distracted, disinterested, or just tired, it can be a real chore to pay attention. I'll bet you've walked out of a class lecture once or twice feeling as though you didn't hear a single word that the teacher was saying.
Well, reading a novel works the same way. If you're focused and really "into" the story, then you'll find that you can read quite quickly and still keep up with what's going on. If you're just moving your eyes mechanically across the page while your mind is on something else, then the important points of the book aren't going to stick. So, the trick is to teach yourself to really focus while you read.
One great way to help boost your comprehension is to engage more of your senses in the process. At the end of each night's reading, pause to jot down a few notes about anything that seems important. Were there new characters introduced? Did anything dramatic happen? Were there any words or passages that you didn't quite understand? Often the act of writing down those simple facts will help cement them into your memory — even if you never actually refer back to them.
You can engage your sense of hearing by talking to someone about what you've read. Call up a friend from the same class and briefly review and quiz each other about the night's reading assignment. Tell your parents about the book over dinner. Even griping about how boring the book is can help you remember it, especially if you cite specific examples!
If you're a visual type of learner, you might try making some rough sketches of the major characters. Combine your drawings with a few notes about each character's major traits and key events, and you'll have a great resource to study from later. Don't consider yourself artistic? Stick figures will do just fine.