there is no 'best' book imo. i just used a variety:
cambridge, phoenix, excel, coroneos, the leading edge and then past hsc
i know its a lot, but i've come to realise its better if you use a variety of books.
In year 11 i only used cambridge thats it, and i didn't recieve a good understanding from it. But now, I am starting to grasp things better and find it easier than when i first started taking 4u.
out of those resources, i'd have to say phoenix for basic understanding then past hsc will be best.