How can I use book as folder name and expect it acting like course or others?

[ikingye]

When I set folder name course, everything is as expected.

course
├── _index.md (type=docs)
├── course1
│   ├── _index.md (type=book)
│   └── example1.md (type=book)
└── course2
    ├── _index.md (type=docs)
    └── example1.md (type=book)

http://localhost:1313/course/

http://localhost:1313/course/course1/

http://localhost:1313/course/course2/

---

But, When just change folder name from course to book, things goes wrong.
How can I use book as folder name, and expect it acting like course?

ps: I find that, setting the folder name as books or others, it act just like course. Why book acts like that?

book
├── _index.md (type=docs)
├── course1
│   ├── _index.md (type=book)
│   └── example1.md (type=book)
└── course2
    ├── _index.md (type=docs)
    └── example1.md (type=book)

http://localhost:1313/book/

http://localhost:1313/book/course1/