The letters on the front of the drawers tell you all the book cards that are filed inside. Books are filed alphabetically by the first letter of the title.

Locate the drawer where each book would be filed (i.e. find the “WX” drawer for Walden) then open the drawer to reveal a letter inside. Write that letter next to the book in the police files to get a secret message.

Opening the drawer for each book title will get you the message “LIFT TOP EDGE OF FOX.” When you lift the top edge of the frame with the fox, it slides up to reveal a letter that Peacock hid inside.