I think New Ideas of Applied Inquiry and New Methods of Applied Inquiry (both from Hubei People's Publishing House) are good. If you still feel simple, I suggest you buy a new helper or Aokao (East China Normal University Press).

I have all these books except The New Helper, which makes it much easier to learn math at school.

(ps: Because I don't know your situation. But I still want to say that the Olympic math test may be more useful for people who want to participate in the competition. You can learn about it. )