Sakutaro, Mitsuki and Masato find the logbook near the corpse inside the girls' lavatory. It provides you with the code to open the reference room. The group goes to check inside classroom 3-1, seeing a note on the the desk telling people to bring it back.
After you get the Goya Statue inside the reference room Mitsuki will take the logbook from Sakutaro, saying they should bring it back since they don't need it.
7/1 [Pool Opening] The pool was finally open today! Everybody put on their bathing suits and went for a swim, after some stretches and such. The water was so cold, and everybody was really having a great time together. Until somebody slipped and fell and hurt himself really badly, anyway. Cracked his skull and suffered brain damage. Seems he can’t talk like he could before, and can’t even feed himself anymore...
8/ 2 It’s sum r v ti but I have school today. At least 4 students forgot their disaster hoods. The teachers got re y mad at th m. After an anno , we had to go under our d And we had to move around o wearing our disa hoods, I guess to see how they fel It’ so hot though! Did the pri really have to make such a long speech? 3 minutes! Some people even fainted! Then we heard the 2 announcement and went back to class. And we ate our emergency rations. There were 2 students unaccounted for, so we split up and searched for them.
Today was the memorial service of the three children who were murdered. We all sang for Kanno, Yoshizawa, and Tsuji. There were lots of mean comments from parents while the principal was speaking. They told him to go to hell, and take responsibility, and stuff like that. But if we told our principal to go to hell, our parents would scold us!
Significant plot details end here.
The logbook belongs to class 3-1 and seem to be written in 1973, since it mentions the memorial for the Ryou, Yuki and Tokiko.
The code to open the reference room is 824322, all of the numbers appear on the second entry of the book.