it is random, so sometimes you cannot get a good grad,
The bookmark is a particular color,
get all the books adjacent to bookmarked book to that color before getting the book with the bookmark. the more ajacent books the better the grade, and the more points you get.
for example, if you have the bookmark of yellow with two books above it and one bellow it. then get the three surrounding books yellow before getting the bookmarked book.
if there is a strait line of books next to the bookmark and you do not get one of them in the middle then you will only get credit for the books closest to the bookmark that are the correct collor.
I hope that helped
Monty
