I know this is a FAQ, but I've checked:

$config['calibre_directory'] = '/storage/Media/Books/';

File /storage/Media/Books/metadata.db not found, Please check

ls -lZ /storage/Media/Books/metadata.db

-rw-r--r--. 1 poc poc unconfined_u:object_r:unlabeled_t:s0 4128768 Aug 6 11:58 /storage/Media/Books/metadata.db

Suggestions welcome.