When is a comic not a comic? When it's a book.
I have nothing against comic books, except that sometimes they can be a bit violent and they're not always terribly well-written. Fortunately, there is a healthier alternative.
These books have superheroes and illustrations like comic books, but they're non-violent and the vocabulary is age-appropriate.
Perfect Man - Michael Maxwell McAllum suspects that his teacher is Perfect Man's alter ego. A heartwarming book with a wonderful ending. My favourite.
Atomic Ace (He's Just My Dad) - Atomic Ace misses his son's recital because he's battling crime. Even superhero dads mess up. The main text is written as a poem; the illustrations are very similar to a comic book.
Max - Born into a legendary family of superheroes, will little Max ever develop superpowers? The illustrations, and the characters, are cute and endearing.
Superhero ABC - From Astro-Man is always alert for an alien attack, to The Zinger zanily zigzags through the zero zone. Author Bob Mcleod illustrated Spiderman and Superman comics before this.
There's also Atomic Ace and the Robot Rampage, which is also excellent but might be a bit scary for younger readers. If you know of any other superhero books like these, please let me know.