I agree with chaley that this should be a plugin (it is the only way to show ratings). You would need to configure:
- column for read_count
- column for rating
And the plugin would apply to some books, calculating the rating.
The first "fulfill" of columns could be done with search and bulk metadata edit. For example: read_count:<=2 and rating:false => Assign 1 to rating... And so on.
But, later, for maintaining it, you should execute the plugin any time that read_count changes (I do something similar for books in a series, and it's a manual task).