I am going to be doing 4U Maths starting the coming term. I have a Cambridge textbook that my sister used when she did her HSC, but when I look through it I feel like where the questions may be useful, the explanations are a little vague. I was wondering what books are worth investing in to use with the Cambridge book so I can at least learn the concepts from other books and do the challenging questions of Cambridge.
Unlike the 2/3U Cambridge textbooks, the 4U Cambridge textbook is written by Arnold and Arnold and are very outdated; pretty much reflecting what the 2/3U original Cambridge textbooks looked like. The textbook serves as a decent foundation but more often than not, the questions are polarised in difficulty (either too trivial or too unrealistically hard).
The new 4U Cambridge textbook never got published due to the whole debacle with the new syllabus. It's now regarded colloquially as the Sydney Grammar textbook and is the most superior of the lot (best reflecting the 2/3U Cambridge textbook quality), but it is hard to obtain as you have to order it via your school.
I used the very old Patel Foundations of 4U textbook (i.e. NOT the excel textbook) - it really left a mark because it had a whole bunch of other conics questions, but also a healthy stack of mechanics questions. Rest, just rely on what E6P said.