The Rule
Write at the level your audience is comfortable reading.
Among those who actually read books, reading comprehension is at the 9th grade or higher, but recreational reading is closer to 6th grade or less. People don't like to have to work hard when they're reading for fun. Here's an interesting table that may be accurate.
| Author / Book / Category | Flesch-Kincaid Grade Level |
|---|---|
| James Patterson / Nora Roberts | 4th to 5th Grade |
| The Hunger Games (Suzanne Collins) | 5th Grade |
| Harry Potter (J.K. Rowling) | 5th to 6th Grade |
| The Average Fiction Bestseller | 7th to 8th Grade |
| Average Nonfiction Bestseller | 9th to 10th Grade |
| The Great Gatsby (F. Scott Fitzgerald) | 5th to 6th Grade |
| To Kill a Mockingbird (Harper Lee) | 5th to 6th Grade |
| The Old Man and the Sea (Ernest Hemingway) | 4th to 5th Grade |
| Moby Dick (Herman Melville) | 10th to 11th Grade |
Counter-Example
"The Secret Garden" by Frances Hodgson Burnett
This isn't so much a counter-example as it is a demonstration of what constituted a 3rd-4th grade book in 1911:
She had not wanted a little girl at all, and when Mary was born she handed her over to the care of an Ayah, who was made to understand that if she wished to please the Mem Sahib she must keep the child out of sight as much as possible.
Fleisch-Kincade rates this at 12th grade. The AI rewrite might make one weep:
She never wanted a little girl. When Mary was born, she handed her over to a nanny. The servant quickly learned to keep the child hidden to please her mistress.
Commentary
Mike expresses the theory that the book is run through software/editorial processes to make sure there's nothing in it that's beyond the ability of a 3rd grader to read. Though there is no evidence Cline (or Brown or writes at a higher grammatical level), James Patterson's writers are almost certainly targeting their prose at 10-year-old reading levels regardless of what their own skill level is.