custom_columns:
custom_column_40 is what I showed above
books_custom_column_40_link:
from what it looks like, book 6337 has a value of 3, which is assigned to 6 (which displays as 3 stars).
Not sure why ids 1, 2, and 11 are not present in custom_column_40, or why the values are in a seemingly random order (I can see why the even whole-stars would appear above the odd half-stars, but other than that is confusing).