If you can, get your hands on John Bulmer books. theyre not very common, but really good and have complex stuff in them.
Otherwise, get Riley or Dixon or both. Riley is better IMO, it has more stats, but Dixon is more simple.
Bulmer has really redundant stuff that's way beyond what you need in the HSC tbh. Dixon's and Riley's textbook are a great combination. Dixon for definitions/general concepts, then Riley for the detail. Also, leading edge handouts etc are great too.