Mabinogion

The Mabinogion is a book based off a blog created by Mori Summer, which describes magic.

Sanae Dekomori and other Chuunibyou patients have copies of it. Yuuta Togashi sealed away a copy in a box at the bottom of his closet.

One of Shinka Nibutani's main motivations is to destroy all copies of the book, as it embarrasses her.