From the Super Mario Wiki
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The Big Bookcase is a scrap found in Paper Mario: Sticker Star. Mario is required to retrieve it in The Enigmansion by traveling to the the cabinet in the middle room on the second floor, defeating the stack of 82 Boos in the area, and examining one of the cabinet's doors to obtain it.
Mario must then travel to the library to the right of the mansion's front entrance, then place the Big Bookcase into the blank area, allowing him to access the basement of the mansion.