When I started to read, I initially non-fiction. Specifically books on films and horror/monster/fantasy films. Before that, I looked at the pictures. Dinosaurs, race cars. Before 1970, I did not know books on monster movies even existed. I started reading fiction in high school, but voluntarily reading fiction when I was around 17. I recall voluntarily reading The Hunchback of Notre Dame when I was about 15. It took me a long time, many months, to read it. I bought several monster books around then from the second hand store. Frankenstein, Dracula, The Invisible Man, Dr. Jekyll and Mr. Hyde. They are still sitting on my bookshelf, unread. I found it very difficult to get interested in the story. I literally just told someone (through a text) that I should read those while I still can.
Why is autism associated with fiction? Maybe because it is more of an internal creation, the world is more internally created. I don't know. ???? That is pure speculation. Horror movies existed in my head, and on film, but most films existed in my head when I was a teen because I mostly knew the pictures from books, Illustrated this, Pictorial history of that, etc.