Excellence in Learning and Practice is a textbook integrating learning, practice and consolidation. The so-called "closed-book" refers to the "closed-book sales" that has arisen in the teaching auxiliary book market. You have to buy it in a specialty store. It's hard to buy it because it's not available anywhere else. Closed sales is actually a combination of market agency system and direct sales method, and it is a way for channels to achieve a win-win situation in a market with stricter mutual commitments and mutual constraints.