I looked at all the eco textbooks during my HSC (2006).
I'd recommend to anyone - Dixon and Bulmer.
Riley as a 3rd resource if you’re really keen, otherwise don't bother.
The other textbooks are useless, even Riley is pretty mediocre.
Dixon's good to get an understanding of the concepts, though its simplified and Bulmer will give you a much better understanding of the concepts involved, it's a much better critical analysis. That said Bulmer tends to stray off topic onto other interesting areas that aren't strictly relevant to the syllabus. So make sure you read it with a syllabus in hand.
Dixon is written to the syllabus dot points, so no worries there.
Oh the little leading edge answer books things aren’t bad either. I mean you shouldn’t really need them, but their well written. I’ve got the whole collection, which I was planning to flog off but their useful since I tutor now.