The Art Book is awesome. It basically enlarges certain important scenes, and because of that, things that are sub tle or seemed like not important, now shows up the details.
Yuzu would buy a manga and then sneak it all the way back home and put it in what she thinks is a secure place which Mei cannot find, Mei? Simply read it in the library back and forth and never takes it home.
Example 1. Yuzu has a meltdown due to Kana and Manami's homophobia. She decided to hide in the library. Walking past Mei, Mei told her to stop. What Mei pulls out from the shelf was the book, "The introduction to the psychology of Love"
Example 2. Then about one year has past and it is Citrus Plus. Mei is in the book shelves area and caught Harumin. Mei already pulled out and holding a science book. But she was also in the process of pulling out....yup, "The introduction to the psychology of Love".
Those are small incidents and most of us would not care. BUT the two incidents are almost the same, hence I think that Mei read that book bit by bit. So in a way, Mei is actually very serious about being with Yuzu. If Yuzu lives by that dating book she made, then Mei must be secretly reading the psychology book.
All of these would be unable for us to notice, until, in this case, a blow up panel so you can read everything no matter how minute it is.