Norton Juster's "The Phantom Tollbooth" - if that hasn't already been suggested? I read to/with DS when he was 5. He thought it was hilarious, and read it on his own at 6.

How about biographies for kids? DS enjoyed ones on Ben Franklin, Lincoln, and Washington.

And this is hardly literature, but the Beast Academy math books. They're written comic-book style, very engaging. DS began with the 3rd grade series at 6, now onto 4th grade (second book in 4th was just released). DS has read each one several times, and apparently it never gets old!

http://www.beastacademy.com