"Math for Everyday Use" by Alex Biddle. It's a generic math textbook that is mostly unremarkable, except for its small size: 6cm x 6cm x 2cm.
If you were wondering what Amane was holding in this picture, this is the book. It's a real book, and it was great reading material back in the 1980s before the advent of smart devices that fits in people's pockets. These days it seem to enjoy a better life in miniature photography.
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