Here is a chain that uses templates to convert url/uri identifiers from three public-domain book websites (
Project Gutenberg,
Standard Ebooks, and
Global Grey) to site-specific ones. Mostly because it looks nicer in book details.
Before installing this, go to Preferences > Look & Feel > Book Details > Create rules to convert identifiers into link. Then add these three rules:
Project Gutenberg
Key: gutenberg
Template: https://www.gutenberg.org/ebooks/{id}
Standard Ebooks
Key: standard
Template: https://standardebooks.org/ebooks/{id}
Global Grey
Key: gg
Template: https://www.globalgreyebooks.com/{id}-ebook.html
Note that it uses the publisher field to identify whether to convert the IDs.