I recently discovered that iTunes has
an API that lets you search for books available on the Apple Books store, and makes high-resolution covers available for them. And so taking the idea from the Kindle hi-res covers plugin I decided to make this API available as Calibre plugin.
The nice thing about the API is that you don't need to know any Apple Books-specific identifier for a book, you can just search by title and/or author. The one downside is that the store is country-specific, so you may need to configure your country in the plugin's settings. I've also added an optional secondary country setting in case you often want to search for books from different countries. Unfortunately there's no way to just search all country stores.
If available in the metadata the plugin will first try to look up the book's ISBN directly, and if unsuccessful it will fall back to the search mentioned above.
Note that the API documentation says that there is a rate limit of about 20 requests per minute, although I haven't been able to trigger that yet. Configuring two countries means there will be more requests per search, so there is a chance that this will hit the rate limit earlier.
The source is available here:
https://github.com/majutsushi/apple-books-covers