who is the developer and how do I get to my kindle books already in my computer

You can download the current version of the Kindle app from Amazon. Their process for downloading is a little unusual. First go to https://www.amazon.com/Amazon-Digital-Services-LLC-Download/dp/B00UB769KC?th=1 and "buy" the app (it is free). After checking out you will get an email; click "Access your content" in that email. After it downloads, go to downloads and open the download, then slide the app to the folder as indicated by the arrow in the window that opens.

The Amazon Kindle application is in the Mac App Store and is the same version as on the Amazon Kindle site.

😎

That's a better idea and easier to install! I rarely think of the Mac App store -- I'm slow to adapt to change apparently.