Fish book, wild letter,
Name: Yuxin Yanshu
Pinyin: yuxün yàn sh
Explanation: fish letter goose book: ancient refers to letters. The latter is often used to refer to letters.
Source: The poem "Yi Dao" by Liang Wang monks and Confucianism in the Southern Dynasties: "The ruler is in the fish intestine, and the heart depends on the goose's foot."