Some books in the history books say ... book shadows. What does "book shadow" mean?

"Book shadow" refers not only to the cover pattern of a book, but also to the inside page and back cover. Generally speaking, the function of book shadow is to show and introduce the style of a book to readers, especially for the display of rare books of ancient books. The book Shadow is published now, mainly for the study of edition bibliography. The forms of ancient books, such as lines, fonts, margins, ink colors, Tibetan seals, inscriptions, etc. If it is described in words and detailed, it is still an armchair strategist. You'd better take a look with your own eyes. As most of my friends who are not in the field of edition bibliography, it is very interesting to enjoy books and videos.