Go to Taobao, after all, the math contest is not a college entrance examination, and there are no reference books in general bookstores. I used to recommend a set of books called Orsay Classics when I was engaged in mathematics in the National High School League. This is what I did. I was lucky to win the first prize. The main explanation is that the answer is very detailed and good. We must stick to the game and not give up halfway. I wish you success!