If you happened to use Kindle eBook reader on Android and wondered where it stores books bought in Amazon, then it's here:
File extension used by Amazon books is .prc
You can copy the file to PC using command like:
adb pull /sdcard/Android/data/com.amazon.kindle/files/B00FOT936Y_EBOK.prc /tmp/
You can also push .prc books to Android similar way:
adb push /tmp/mybook.prc /sdcard/Android/data/com.amazon.kindle/files/mybook.prc
If you bought the book from Amazon then it will be secured with DRM. This is easy to remove, some people do it even though illegal.