The act of reading is a skill that needs maintenance. It doesn't matter much whether it's fiction or nonfiction. Harry Potter is a bit of an unfair example as

The act of reading is a skill that needs maintenance. It doesn't matter much whether it's fiction or nonfiction. Harry Potter is a bit of an unfair example as it's intended as a children's book, they still need to build up the skill. But more importantly, good fiction (and other art) _teaches about the human condition_. I have learned a lot from Vonnegut and Pratchett even though everything I read by them was fiction.