Grades 4 through 6 are a golden opportunity to get kids hooked on reading. Children at ages 9 through 12 are ready for books 20,000 to 40,000 words long.
They aren't ready for Anna Karenina or Moby Dick, but they can certainly read classic books, like The Lion, the Witch and the Wardrobe by beloved author C. S. Lewis and science fiction adventure stories, like A Wrinkle in Time by Madeleine L'Engle. They might even be interested in contemporary children's fiction, like Roller Girl by Victoria Jamieson.
It's a great opportunity to challenge your reader with educational reading, too. They can also read supplementary textbooks like Everything You Need to Ace Science by Workman Publishing and the editors of Brain Quest.
Here is a simple rule to follow when you don't know which book to buy for your 9- to 12-year-old: buy as many as you can! When you encourage children this age to love reading, books will become more cherished than toys.