Sync books bought/downloaded from the internet and added to kindle

[mehmedmaljoki]

Is your feature request related to a problem? Please describe.
The plugin only synced books bought on Amazon but not those added afterward. For example, if you bought the .epub somewhere else, you can add it to the Kindle app, but the book is not synced with the plugin.

Describe the solution you'd like
The sync should be possible for all books.

Additional context
We can work with the other options by exporting files but it's not that comfortable