Thank you for all the great tips.

The fade to black sounds good but it seems completely sold out.I will experiment with the solargraph but it is not very usefull to me when it disappears in the dark.

To be more precise, my plan it to add the envelop with light sensitive image with a publication. So people who buy the book (the edition will be 70) can choose themselffs if they want to see the image, but by seeing it they will destroy it, or not see it and keep it intact.