Graduate students in economics generally take three public courses: Mathematics (3-3), English and Politics. You can easily find learning materials for these three public courses, and there are many introductions on the postgraduate entrance examination forum. Just look at it.
The problems of specialized courses should be specific to each school and major. The school majors are different, and the bibliography of professional courses will be different. Generally, the professional bibliography of each school will be published in the school official website at the end of August and the beginning of September, or you can refer to the bibliography of previous years. Generally speaking, there has been little change.
If you are worried that the books of specialized courses are too difficult to learn, you can first find some related basic books to read, such as microeconomics, macroeconomics and so on. The recommended course is a concise course of economics published by Fudan University in Shanghai (compiled by Yin Bocheng), with two supporting workbooks, which are moderately difficult and highly appraised in all aspects.