[tingley]

A hollowed-out book that contains something other than pages is literally a fake book. It looks like a book, but it is a fake. It is not a real book.

[aaron695]

I'd say a repurposed book.

It's a art piece that has been changed to another type of art.

Book -> performance art

Although since inside is a written story, I'd argue it's still a a book, unless it leads to further clues then it becomes performance art.

A good example of a fake book is a book made of plastic which is then used in ikea or has a hidden void.

[anigbrowl]

A hollowed out book does have a hidden void. I'm a pedant too, but I try to avoid contradicting myself. Your assertion that the contents constitute a written story is unsupportable, given the singular absence of declarative sentences or anything resembling a narrative.

[stuckagain]

Not to be confused with a Mock Book.