MacOS and Kindle file Location Changed?
Using Kindle v1.17.0, Kindle content is stored in the location set under Kindle menu > Preferences > General tab > Content Folder. (The default appears to be ~/Documents/My Kindle Content.)

If you are using Kindle for Mac from the Mac App Store, it has not been updated in nearly a year, and you should switch to the version downloaded directly from Amazon.

You can download or update Kindle for Mac by getting it directly from this Amazon site: https://www.amazon.com/gp/kindle/mac/download. (Note that clicking that link will initiate the download.)