We have done very well with comic books-- old Disney ones have clean content. And with the magazines from Cricket. (Ask magazine is mostly in comic format but treats interesting topics. Muse is aimed at older kids but DS5 loves it. Muse has relatively short articles, but at a nicely sophisticated reading level, so endurance is not a big problem.)

And pictorial science books. And a beat-up atlas. And art history books.

There is a large trove of this stuff around and accessible throughout our house. The kids like to stash things behind the couch they're working on...

DeeDee