To unassign an eBook you will need to be in the Reports section of your Reading Planet Online library.
You can filter your reports to a specific book band or pupil group:
Once in your Reports section, you will be able to 'Hide graph and trouble spots' to expand the list of pupils which appear on the left hand side.
To open a pupil report please click on their name, in this pop-up will be able to see their Achievements and any remaining books to be read. Click on 'My Books' and here you will see the option to 'Remove':
Please note: you can only remove eBooks individually from a pupils account, you cannot currently bulk remove