The watch command is useful when you want your book to be rendered on every file change. You could repeatedly issue mdbook build every time a file is changed. But using mdbook watch once will watch your files and will trigger a build automatically whenever you modify a file.

The watch command can take a directory as an argument to use as the book's root instead of the current working directory.

mdbook watch path/to/book

When you use the --open (-o) option, mdbook will open the rendered book in your default web browser.

The --dest-dir (-d) option allows you to change the output directory for the book. If not specified it will default to the value of the build.build-dir key in book.toml, or to ./book relative to the book's root directory.