mdBook/guide/src/cli/clean.md

The clean command

The clean command is used to delete the generated book and any other build artifacts.

mdbook clean

Specify a directory

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

mdbook clean path/to/book

--dest-dir

The --dest-dir (-d) option allows you to override the book's output directory, which will be deleted by this command. Relative paths are interpreted relative to the book's root directory. If not specified it will default to the value of the build.build-dir key in book.toml, or to ./book.

mdbook clean --dest-dir=path/to/book

path/to/book could be absolute or relative.