The girl in the upper right in the last panel seems to be Yuubari, as she is also often aimed at for flat-chested jokes, and she has a ribbon and a ponytail. I thought it was Kumano at first but her head accessory is metallic and can't fall down like this. So I added the tag, but don't hesitate to correct me if I'm wrong!
Secret of Big BoobsThe book?Oh my.Keep it up.That's good, good for you.That's a difficult book you're reading there.How about you read it once in a while too?The mood, I mean.