Source "White Tiger Tong Five Classics": "What are the Five Classics? Also known as Yi, Shangshu, Shi, Li, Chunqiu. " Song Zhuxi took notes on four books.
The Four Books and Five Classics are the basic bibliography of Confucianism after the Southern Song Dynasty, and they are also required books for Confucian students.
Personally, I think this is related to what Zhu Cheng's Neo-Confucianism advocated at that time.