As of the time stamp on this posting, there has been and still is nothing to be seen of the User’s Guide at:
https://guide.pressbooks.com/
– which instead goes to a blank page at:
https://pressbooks.pub/wp/wp-admin/admin-ajax.php?action=mercator-sso-login&host=guide.pressbooks.com&back=https%3A%2F%2Fguide.pressbooks.com%2F&site=4748&nonce=81356b66df&fragment=
This link hasn’t changed for me, so if it’s not resolving correctly, ???.
J. at a less remunerative public library link in CO
Not sure what happened there @Jay_P_T but the guide appears functional for me when checking with various devices and browsers. If it’s still down for you, will you try clearing your browser cache?