The problem? My son got bored.
He'd read about animals, robots and knights. Then he read about mars, dinosaurs, and volcanoes. Then he read about more animals, more robots and more knights. Then he became bored. the problem was that he'd already looked through most of the books on the subjects he was truly interested in and he wanted more. It got to the point that he no longer wanted to go to the library. He stopped asking for books because he didn't see anything new.
The solution? The adult nonfiction area.