@sondr3 - the authentication is working, at least it was for me just now using Chrome.
All I did was:
(1) I was signed into goodreads correctly with my web browser open. You don't "have" to do this first since you would get asked to sign in if you omit this step.
(2) Since I knew my account was already authenticated, I clicked on my user name on the website, chose "Edit Profile", clicked on the "apps" tab, scrolled down the bottom of the page and revoked access for the "no application" entry (which represents this plugin). If you have never successfully authorised this step does not apply.
(3) In calibre brought up the configuration page for this plugin and clicked on the Authorise button.
(4) That displayed the webpage I expected in Chrome where you have a button to allow access for the plugin which I clicked
(5) Back in calibre I clicked on the "Yes" in response to the prompt confirming I have clicked Allow on the web page
That is it. From now on if I got back in the website via Edit Profile/apps I can see the "no application" entry appearing at the bottom of the page as an authorised application, and adding books to shelves etc just works...