Most criminals who are locked up as a punishment or to await trial are just confined in regular cellblocks or jails, but not all of them. Some are so dangerous that ordinary prisons just will not hold them. Others have secrets that cannot be allowed to be spread, or their crimes are so dreadful that they deserve a greater punishment then normal incarceration or even execution. In yet other cases the ruler had a nasty sense of humour...
You find a book in the library, then when you start to read it, seems to change from pages of words to pages of images that seem to live and move. If you touch the page after the images appear, you travel into the story and now live in that world.
There is a book on that world, that leads back to this one.