1. I'm getting the impression from my teacher that I cannot solely rely on one source for this subject (I've been encouraged to use Nature of Biology, Biozone and Biol Notes simultaneously, to give you an example). Is this really necessary?
OH YUS!!! I found that the Nature of Biology textbook was not the most reliable resource as it often skimmed through necessary information and went into too much detail with stuff that wasn't directly related to the curriculum (far too many useless examples, etc.) Personally I found it a whole lot better to rely on Biozones when preparing for SACs/exams as they encouraged you to think about the information and provided far more in-depth and relevant descriptions. Biozone saved me countless times in year 11/12.
2. Is it just me, or does the Nature of Biology textbook have a lot of irrelevant information? Also, I've found that the three sources mentioned above seem to cover different things within each sub-topic. How much detail do I really need to delve into?
Yes you are very correct. You don't need to delve into too much detail, just the basics, however Nature of Biology tends to skim through a lot of important stuff far too much and provide an excessive amount of superfluous examples which you most likely won't need (though I would still recommend you read them anyway for a bit of fun facts and just in case they sneakily put a question related to them in a test or something). The diagrams are especially rushed and I would definitely recommend supplementing Nature of Biology with Biozone and the internet.
~Bel V.P.