Take notes. It is possible to read a textbook for fun, but the practice is not very common. Most people read a textbook because they need to get information, and textbooks are an excellent source of concentrated, clearly-organized information on many topics. To get the most from reading your textbook, have a notepad open beside you while you read.
Set a pattern. Read one paragraph at a time, then stop and make a note about what that paragraph said. Try to put it into one or two quick phrases or sentences.
Review your results. At the end of your session, you'll have a personal copy of all the information you need. Read over it to ensure that everything makes sense to you.