Originally Posted by Berzelius
Try doing it via Calibre. I've just moved a couple of non-Amazon books to the Android Kindle app on my phone. When you connect your device to your PC Calibre asks for permission to manage it. After agreeing you just need to select the books and click on the Send to Device button.
Thanks Only tried Calibre with my Kobo so far.
Got such a lot of books now lol