If you've never done this, it will take a while for iCloud Drive to sync. You will not see the Books folder there, but you should see some things just to confirm the iPad is connected. The iPad will appear in the LHS navigation bar. Remember, this is for Big Sur and other recent versions of MacOs that don't use iTunes for this. Plug the iPad into the Mac and open Finder. On the iPad I was working with this was already done so I can’t detail the steps, but they are in Settings. On the iPad enable iCloud Drive and enable it specifically for iBooks.

Step 1: Sync books from iPad to MacOs using iCloud Drive We’ll copy them to the Mac, retrieve them from the secret-double-hidden folder they are in, convert them into compressed single-file ePub format, convert those using third party software Calibre into to MOBI files that Kindle can read, and move those to the Kindle. The iPad stores books in ePub format, uncompressed and expanded into packages or folders. This assumes some working knowledge of settings in iPad and Kindle, good knowledge of Finder advanced features, and knowledge of how to get around the system in Terminal and run shell commands and simple scripts of 2 or 3 lines from the command line.Once you've completed this you'll know what to do with PDFs. This focuses on epub files, and ignores PDFs and other things.If it's important to you to migrate every book (which may not even be possible) you'll have to pay close attention and read between the lines. The process below does not document the errors, it only documents how to blindly migrate the books that migrate easily by following these steps and ignoring errors. If you find errors below, feel free to correct them or comment and I will. The process was long and arduous and I undoubtedly did not capture every step perfectly.

Older versions of MacOS (IDK which ones) used iTunes, and this answer doesn't apply to those, or to Windows or other computers. I migrated via a Mac computer, running Big Sur.Migrate from iPad to Kindle via MacOs Big Sur with iCloud Drive and Calibre Caveats

Upload epub to kindle