This is nowhere near as high-quality as many of the books listed, but right now DS7 is greatly enjoying Cressida Cowell's series that starts with "How to Train Your Dragon by Hiccup Horrendous Haddock III". The series is about a young Viking hero (Viking and hero used very loosely), Hiccup, and his various adventures/misadventures. It's got a good dose of body humor, though not overwhelming to the same extent as Captain Underpants (which DS refuses to read for reasons unknown to me). Hiccup, his dragon, and his best friend are far from the accepted norms for his tribe, but they do manage to succeed at things, eventually, in their own way.

Anyway, DS is enjoying it and I like that - it's a little longer and more complex than the books he's been subsisting on for fiction. I like that it's published in hardcover and the typeface is easily read - that keeps it from setting off the "It's too hard for me to read" alarms in his head. Not sure of the reading level - maybe 3rd/4th grade? DD12 likes the series too and sneaks the books off to read as well.