https://git-scm.com/docs/git-submodule

When in doubt:

    cd /var/www/pressbooks/wp-content/plugins/pressbooks
    git submodule init
    git submodule update

Add a submodule:

    cd /var/www/pressbooks/wp-content/plugins/pressbooks
    git submodule add ssh://bla submodule_dir
    git submodule init
    git update

Update submodules to latest version:

    cd /var/www/pressbooks/wp-content/plugins/pressbooks
    git submodule foreach git pull origin master
    git commit -am "Updated submodules"