This book has already been published for a few months and it is one of the annual anthologies presented by pixiv. This one is dedicated to girls only illustrations published during the last year in the website.
They held a contest to decide wich illustrations will be featured in the book and one of the members of C★R, Aiwa, was choosen. The theme of said contests was “girls painting”, so all the illustrations in the book concerning the contest follow this theme.
You can see her illustration in the following link:
「青」 by Aiwa: [pixiv link]
It was her first time in a Japan publication and we hope it will not be the last.