Introduction to Sizhi Bookstore: It is behind the Jingcheng Hall in Danbo, the Summer Resort. It is a large hall with five rooms. Emperor Kangxi once named it "Yiqingkuang", and in the 51st year of Qianlong (1786), he added the title "Four Knowledge Library". The Four Knowledges are taken from the "Book of Changes", which means "a gentleman knows the subtle, the obvious, the soft, the strong, and the hope of all things". This hall was where the Qing emperor changed his clothes and rested before and after the grand ceremony. There is a semi-enclosed corridor around the courtyard, which divides the limited space into several parts. Each part forms a separate scene and sets off each other, strengthening the spatial layering effect of the courtyard.