I recently downloaded a book in .epub format and wanted to transfer it to my Kindle. Since that format cannot be directly imported, I needed to convert to another format. An acceptable format is .mobi. Kindle makes a free app called Kindle Preview to allow ebook producers to preview what their books in progress look like on a Kindle. The app also converts from ePub to mobi. The ePub format was dropped into the Open Book window and after a while was readable. Once imported the book could be exported to .mobi format.
I tried to email the book to my kindle (after finding out the kindle device email address using the Manage your devices option on Amazon, then clicking on Preferences). But the book was too big to send so I discovered that I can simply connect the Kindle reader to the Mac using the usb charger cable. The .mobi file was put into the Documents folder and it then appeared on the Kindle.
Update: To input a .mobi file into Kindle, download the file on the iPhone. I navigated to glutenberg.org and downloaded an Agatha Christie novel (Ebook-No. 863). The file is placed in the Files folder. I clicked on the .mobi file which brings up a blank window. I then clicked on the send icon and chose Kindle. I could then enter the title and author and select Send to Kindle.