Captain Underpants is a book series by Dav Pilkey, and it's better than it sounds. Or looks. These are silly superhero fiction, with books composed of a short stories interspersed with illustrations and comic book pages; while it is very much targeted at kids, it is amusing for all ages.
George Beard and Harold Hutchins are a pair of elementary school troublemakers; they like pulling pranks and writing comic books. Mr. Krupp is the mean principal; he should be mean to George and Harold, they are destructive and thoughtless and only want chaos. But also, Mr. Krupp is a crappy person in his own right. In the battle of Wits Vs. Krupp, the tide is turned when one day George and Harold buy a hypnotism ring and hypnotize him to be Captain Underpants -- a joke superhero that involves Mr. Krupp stripping down to his underwear, wearing a curtain as a cape, and running around trying to be a hero. But, one way or another, crazy things keep happening, and Captain Underpants keeps saving the day (well, mostly because Harold and George save his bacon, but still).
The general format is a standard introduction, done in comic book form (as written and illustrated by George and Harold, rife with spelling mistakes), a number of short and heavily illustrated chapters, another comic book break, and a few pages of "Flip-O-Ramas", a set of two pages that you flip back and forth quickly to make a moving picture (usually a battle scene). There are lots of references to underwear and toilets and terrible cafeteria food; there's a large cast of mean and dumb teachers, and the terrible things that happen to them; and the whole thing is dumb. But also, there are plenty of good jokes, including ones that kids may not get the first time through, and some they may not get at all. These are books designed to be a fun read-aloud, to have high re-readability, and to exceed the promise of the titles.
The series is surprisingly long:
... and three activity books, three spin-off novels (The Adventures of Super Diaper Baby, The Adventures of Ook and Gluk: Kung-Fu Cavemen from the Future, and Super Diaper Baby 2: The Invasion of the Potty Snatchers), and a spin-off series, Dog Man, which currently has ten books. So, be careful about getting your kid hooked on these.
If your kid does get into the series, tho, that's not a bad thing. These books are cognitively engaging. They have a comparatively low
reading level while still containing a lot of harder vocabulary, complex sentences, and simple inferencing challenges that kids often don't see in books written for younger kids. And, of course, they're fun, and kids need clear, strong indicators that reading can be fun, no matter what tripe their evil teachers might force upon them.