It would seem that i was hasty...
When using Import List, your Title and Author fields MUST be populated, i.e. Non-blank. This i did not know and accidentally found this out. So, for future reference:
It is possible to update your metadata using a CSV file by using the Import List plugin. Your CSV file May have empty cells/entries, just not in the Title and Author categories, as the plugin cannot read the entry as a book then.
Hope this helps someone.