

Your Send-to-Kindle e-mail address is a unique e-mail address assigned to each of your Kindle devices or free Kindle reading apps upon registration. If you're unable to transfer books to Kindle via USB, you can still send documents from Mac to your Kindle eReader, Kindle Fire, or Kindle reading app by e-mailing them to your Send-to-Kindle email address. Send Books from Mac to Kindle via Send-to-Kindle Service

In this occasion, please continue to read to learn another two methods to send the files to Kindle Fire wirelessly. Note: Some Kindle Fire tablets cannot be recognized as USB drive by the Mac computer, so you'll fail to send books from Mac to Kindle via this method.

Then just copy and paste the books to "documents" folder. Just connect your Kindle device to the Mac computer with the USB cable and it will be listed under the "Devices" tab on the "Finder". When talking about transfering files to Kindle from Mac, the first method that comes to our mind would be using USB connection.
