Many are adapted from popular fiction books.
Example
Many ELT publishers produce graded readers for a wide range of levels.
In the classroom
Graded readers can help learners build their confidence in reading, and they can make a useful addition to exposure of authentic reading texts, even at very low levels. Activities with graded readers include imagining and creating the past lives of characters, dramatizing key scenes, and writing letters to the author.
Further links:
https://www.teachingenglish.org.uk/article/victoria-boobyer-a-why-how-graded-readers
https://www.teachingenglish.org.uk/article/using-graded-readers