[haertig]

I don't know if just putting a book in the correct folder is enough. I can't say specifically for Kindle books, but I just did a test yesterday, coincidentally, for an Overdrive-borrowed audiobook.

Overdrive normally downloads these audiobooks to:

/root/storage/sdcard1/Android/data/com.mobile.android.mediaconsole/files/Overdrive/[bookname]/*.mp3

I routinely move them to /root/storage/sdcard1/Audiobooks and then access them from there using the SmartAudiobookPlayer app. I do not use the built-in Overdrive listening app. Anyway, on a whim, after moving the audiobook files I decided to immediately move them BACK to the normal Overdrive directory. Just to see if the audiobook would reappear in Overdrive. It did not. So there is something more needed than just putting files into the correct directory for Overdrive to recognize them. Maybe it's the same for Kindle eBooks and the "Books" app we're discussing in this thread.

Note: Android usually blocks you from freely moving files like I do above. But you can do it with dedicated apps that you manually allow permissions for. I use the "ASUS File Manager" app for this purpose (it's not just for ASUS branded devices, it works on FireOS tablets provided you've already installed Google Play Store where you can install the ASUS app from).