It is easy enough to copy ebooks in their various formats to the kindle folder on an Android external memory, however the Kindle app doesn't always recognise them.
After scouring the internet I found a simple solution;
- Download the Calibre ebook manager.
- Open Calibre, and use the Add books button (top left corner) to select the required ebooks.
- Use the convert tool to convert the ebooks into .azw3 format, you can also customise the output resolution to suit your own device as well as many other options
- Copy the .azw3 files to your microSD/other storage device, placing them in the kindle/ folder.
- Open the kindle app and use the sync button (top right hand corner) to search for new ebooks. It may take a few minutes to update, depending on the size and number of books added.
Also note that Amazon Prime members can only make use of the free book 'lending' scheme if they are using a Kindle device.