Your reading comprehension leaves something to be desired. The MC — after the first of his two wiggings-out — subvocalizes “I'll just put it ba—”, so obviously restoring it to her possession is not a given with this coward, who is instead planning to leave her as exposed as he found her. Then, his having already discovered that he'd stumbled upon private writing, when a piece of papers slips out of the book, he chooses to read it and then wigs-out again, another expression of his cowardice (coming after his wilful intrusiveness). He shoves the book back into the hole whence it came in order to avoid trouble, and then he quite literally runs away from the situation.
*Before you read this, I made an edit on the bottom of my previous reply. You should probably read that, if you don't, then my change in wording here might not makes sense to you.
Holy fuck man, you really don't want to lose huh? Even going so far as misrepresentating the situation in the chapter. It's a good thing that I didn't blindly believe your words and reread the fucking chapter.
Here. Let me slowly and step by step lay out the stituation in the chapter for you. Hell, for someone that dares insulted someone else of their reading comprehension, yours certainly aren't any better. Either that, or you are lying through your teeth.
- First the MC discovered the book in page 16. He found out the content of the book in the same page.
- Then the MC protected the book's secret from being found out by his friend in page 17. He found out that the owner of the book was possibly the black haired girl (from here on she will be "the girl") in the next page.
- He spent page 18 and 19 imagining what would happen if the owner was not the girl and he returned the book. From his last monologue, we can assume that he spent the school day waiting for the owner to ask about the book in class.
- The girl is now confirmed to be the owner of the book based from her reaction in page 20. Important Notice, it is now "After School".
- in page 21 he noted that she was nervous about people finding out her book.
- in page 22, after the girl's outburst and subsequent fall into depression, the MC wishes that the girl would just come out and ask about the book already so he can return it to her. As he said, he lost his chance of returning the book to her due to her outburst and fall. If he approached her and told her, or return the book to her, it would just attract too much attention. Attention that the girl surely would not want. IMPORTANT NOTE: Nobody knows that she lost a book. Nobody except the MC. And obviously if people knew that it's a book that got her all hot and bothered, then they would naturally question what was in the book. She wouldn't want that.
- in page 23 she ran out of the classroom. It's assumed that she's retracing her steps, trying to find the book. A fair assumption, given how in distressed the girl was. Of course she would go back around her day trying to find her lost item.
- in page 24, the MC waited for the girl to come back to the empty classroom. The empty classroom being the perfect place to return the book to her, thus he waited there, assuming that the girl would return to search the classroom. He couldn't have possibly chased her, since chasing her would attract attention which the girl wouldn't like. Him waiting there is also a fair. Given that he assumed that she was going around looking for her book, he assumed that her own classroom was one of the place that she would come back to.
Only after the sky is darkened did he considered putting the book back to the desk, leaving the whole returning the book thing for tomorrow.
- in page 25, as he tried to return the book back, he found out another bombshell. That is the girl has been having Yuri fantasies about her own friend.
- Overwhelmed, he ran back home feeling quite obviously unamused, shocked, troubled, what have you. Only after he found out about that last part did he hurriedly returned the book to the desk.
There. That's how the chapter played out. There's no show of cowardice in this chapter at all. The situation played out in a way that did not allow the MC to freely approach the girl. Otherwise he would attract attention far more than necessary, and obviously the girl wouldn't like attention drawn into the item that made her so desperate.
Sheesh. If I were in that situation and found out that one of my classmates had been having Yuri fantasies about another classmate, I would also be overwhelmed. Heck, I should know since my friend made me and my other friend as fuel for her fujoshi fantasies. In my case, I confronted her about it though, since she was the one that told me about her fantasies right to my face anyway. I just told her to stop (or at least change the name and looks) and I went home with a headache. That was too much development for a middle schooler lol. Good times lol.