There's also the classics--my son just found a copy of Great Illustrated Classics The Time Machine on our shelf. For about 2-3 days straight he kept darting up to his room to finish it! Thinking about picking up A Journey to the Center of the Earth for him next.

Any recommendations for science fiction/time travel etc.? He's read Franny K Stein, Magic School bus and some Time Warp Trio. I'm not sure what else to get for him he's still in K, so some of the subject matter might be too mature for him which makes it a bit tricky). Thanks!