The latest update to the Kindle app on Android can now use SD card storage for downloads. When I enabled this, it also offered to transfer existing books to the SD card.
Good news for those who are running out of space on their phone or tablet main storage.
The article below shows screen shots of the new settings page. I've confirmed the new settings are active on my phone.
https://www.androidpolice.com/2018/1...-your-sd-card/