Apparently, there's 2 ways to do it
#1-Connect Kindle to Mac with USB cable. Your e-reader will appear as an external drive, Open drive icon and go to "documents" Drag "documents" folder to where ever you want. Please note-you'll only be copying the files on the Kindle itself, and not the stuff in the cloud.
#2-doing the desktop Kindle boogie.-Get the Kindle Desktop app for Mac. Get the books from the cloud. download by double-clicking on each book, one at a time. during installing the application, Kindle will create a folder for storing all future downloads. here's where the folder should be : ~/documents/My Kindle content. as for transferrring, if the book can be loaned, go to :book details on your Kindle device, and see if the book can be "loaned". If so go to actions, then "loan this title" and fill out the form with the other kindle owner's name and email address, then send now. The book should be available for 14 days. otherwise maybe convert to ePub format, which would be converted by someone else
For one I want to transfer the kindle book from one Mac to another Mac, so your #1 option doesn't apply.
And for the #2 option - there is no "My Kindle content" in my document folder.
When I open kindle on the Mac, the books show as having been downloaded but I can't find them anywhere in any form on my Mac.
I also tried three different Kindle to pdf or Word conversion progrms - can't get any of them to work.
Some can't find the kindle books I bought, others create huge problems on my Mac,endless beachball where I have to restart.
So far this exercise has just been a huge waste of time.
I buy the kindle book, it downloads on my desktop Mac, I can open it and read it there but I cannot transfer it to my MacBook pro.
When I try doing that using the Amazon account all it does is allow me to send a message to tell someone else how great the book is.
One would think after one buys the book one could read it on any of one's devices but that doesn't seem to be the case.
The only partial workaround I have found so far in case anyone else is interested in this topic, is to convert the kindle book to pdf and then one can of course read it on any Mac (or PC for that matter).
Seems to be the only way.
I can't help you in so far as telling you why things are working that way for you, but I can tell you that is not the usual behaviour for Kindle apps on any platform. I can download and read any book from any app on any device, and it works every time. I can buy a book on my iPhone, download it on my work computer, download it on my iPad, then download it again at home, no problem.
Call Amazon support, they're usually pretty good at figuring these things out.
Did Amazon help? I just downloaded the current Kindle reader from the ap store to my mac (had it on my MPB for years). Amazon asked if I was current user and after I remembered my password my library loaded. I picked a unfinished book and it took me to the last page opened. The irony of my situation is the reader isn't opening on my MBP (incompatible software)! Time to update.