Knowledge and methods of senior high school mathematics entrance examination Zhejiang University Press
A Course of Olympic Mathematics (Three Volumes) East China Normal University Press
Test 2: Mathematical Olympiad Series (16) East China Normal University Press
Competition Mathematics Course Chenzhang Higher Education Press
If you want to know the types of questions, you can buy "Prediction Questions of National Senior High School Mathematics League", edited by Cai, Zhejiang University Press.