Definitely, you should get that textbook. If it is affecting your learning and three people really have it than you must. And according to your previous posts you are only doing one VCE subject, getting one and studying two shouldn't be a problem if you have the money. You are still in year 10, so it won't affect your other subjects if you have two. You don't want to take any chances, even if it is for unit 1/2. Getting the textbook will allow you to learn how to study more efficiently and you'll get more used to the thinking processes involved. This will also probably help you to study for your other subjects. You also must top your class, so once you get it make sure actually read it. But before you do that, have a look at its website first, check its sample pages and make sure you are making the right choice.