All what you will do will need check and debug. Description bellow haven't any warranty
But that may be will work if you in file:
Change code at al lplaces:
<img src="./components/com_booklibrary/images/rating-<?php echo $row->rating; ?>.gif" alt="<?php echo ($row->rating)/2; ?>" align="left" border="0" />
Please back up yours file before change.
Also you will need change in all language files word: Rating
But again that not full correctly because - rating will may appear in some other places.
because that please do full check all files.
Or Please order custom version Book library software from OrdaSoft team