My students' writing topic was "If you could send one modern item back in time to one specific era, what item and era would you pick?". Huge range from "bicycle to China, 500 BCE" to "rabid badger to Boris Johnson's college room, 1984".
@fesshole
It would be a book regardless of which era. I'm not sure which right off the top of my head. Although, there would probably be different selections of which book depending on which era/place it was sent to.
Summation: just a regular printed book.