I just tested K4Mac 1.40.1 on Sonoma, and I get the same message on Sonoma when trying to download ebooks as booksies does on Sequoia. So this is not a Sequoia problem. Basically, it seems Amazon has killed K4Mac 1.40.1, books will no longer download and Mac users are told to download v7.x from the Mac App Store. I've started a discussion on the main
Dealing with Kindle ebooks in calibre thread. Perhaps we can continue this discussion there?