Change the last line, putting whatever you want between the quotes. That will be shown if there isn't a read %.
For example (and being a little silly)
Code:
program:
comments = field('comments');
contains( comments,
'(?i).*?last page read: location',
cmp(re( comments,
'(?is).*last page read: location \d+ \((\d+)%\).*',
'\1'),
94,
'No',
'No',
'Yes'),
'Unopened')
Will show 'Unopened' for all books without the annotation.