depends on the subject, and the text book. some text books i saw (but werent used by our school) were absolute shockers... for chem and biol i would have two text books if you can be bothered, biol more importantly. dont bother getting like different summary books, they're much less useful and will all contain the same stuff. text books, however, are not always based on the study design and will often contain either irrelevant things, omit things, or things which VCAA (in their wisdom) thinks would be a surprise challenge onthe exam (but which, if you used that text book, are very obvious). hence i thought for biol and chem, two textbooks ensured i hadnt left anything out of my study.