Thanks to all the advice so far. I will investigate the Database module, which I have never yet tried.
I followed Mary's advice, and allowed forum posts to be rated. The problem I have now is that you can only rate other peoples posts and not your own!
My colleague needs you to be able to leave a rating for inside your OWN comment, as the ratings are not being used to rate the comment itself, but in fact to leave a score rating for a book that has been mentioned.
So for example, I would suggest the book Harry Potter and leave a rating for this book of 5. The next person would leave their comment, and put their own rating in there. Over time, you would have a number of comments per book, each with a rating left by the comment author.
I will continue to investigate this matter!