DD (6yo) is really into superheroes right now. She was very excited to check out a Spiderman book from the library, but she found it a little too easy and wants something more challenging. It was a Level 2 early reader. So I'm looking for books at about a level 3 or 4. They don't need to be early reader books. I'd just like them to be at her reading level.

Oh, she also loves Captain Underpants (ugh, what is the appeal of those books??), although she needs a little help reading those.

Basically, while she loves books, she has been pretty indifferent about reading them herself. So I want to capitalize on this new interest in superheroes to help her enjoy reading.