Just to follow up, the Book Links plugin is currently defunct (see
this thread).
Instead, I will be using
Windows shortcuts for supplementary information (with thanks to Joanna for the idea).
For each book, you can add a shortcut to a folder on your computer. Drag & drop the folder into the Book Details pane, which adds the shortcut as a "LNK" format, and then click on the "LNK" to open the folder.
Note: Even if the folder is moved, Windows can repair the shortcuts automatically. (And if not, this
Google search can probably help.)
calibre 2.57.1 [64bit] on Windows 10 Pro