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Hamartert - if you have the Goodreads identifier (e.g. goodreads:1234567) in the IDs column - then you can use that, it will show as
Goodreads in Book Details.
Or you can insert a url link into Comments (or any other long text column) using the link tool
Hit the button, paste the URL into the URL box, dress it up with a label - eg
At Goodreads, hit OK. And you're done.
For Authors you can put an author specific link (eg their blog) into the Link field in Manage Authors - right click Author Name in Book Details and select Manage Authors
BR