If you're wanting to buy something content-based, I'd recommend the ATARNotes HHD Study Guide. I say this purely because I used it last year and found it absolutely brilliant. Whilst the study design has changed and thus I can't speak on behalf of the new edition, the one I used was the perfect summary of information without compromising on necessary detail. I'm not affiliated with ATARNotes aside from being a regular forum user, I'm literally just heavily impressed with the notes it provided. I did use A+ Notes too, though (I think this is mainly due to the changes that occurred during the study design's accreditation period) I personally found it a bit hit and miss and outdated in some areas. Going to outside lectures can be helpful too. I went to a couple from The Health Teacher's Network which I loved and they provided a really great workbook/summary type booklet. Though, in saying that you don't need these to succeed.
Otherwise for resources outside of commercial study guides and the like, I found looking at websites like the United Nations', Department of Foreign Affairs and Trade's or the World Health Organisation's to be particularly helpful. Also, when studying the Australian healthcare system and nutrition I looked at a few pamphlets like the ones you find at the GP, such as those relating to the Dietary Guidelines. I liked using these because the information is typically aimed at the average person, and thus is simply put. It's an easy way to elaborate on the information you already know as well as expanding your knowledge outside of the parameters of your textbook. Whilst you don't need to know specific statistics in health, I like to think that having some extra knowledge or examples in the back of your mind can give you an edge over some other students (this is not me saying that you should get hung up on memorising the life expectancy/rates of X disease for a particular country). I'm someone who, when learning, likes to look at things in almost too much detail (I'll be damned if I know why it just helps me make sense of everything lol) so I found doing this suited that.
