In Captain Underpants, George and Harold are two (supposed) trouble-makers who get into a lot of sticky situations, not always of their own making. They somehow manage to travel through time, beat talking toilets, acquire a pet pterodactyl, and are responsible for creating the world’s best superhero, Captain Underpants himself. Tra la laa!
With lots of illustrations and light-hearted humor, this series is excellent for reluctant readers.
I recommend this one for reluctant readers because there is no dense prose (but the text is neither basic nor boring).
The 13-Storey Treehouse is filled with unusual things, like a marshmallow machine, a bowling alley, and a swimming pool. Andy and Terry (the main characters) are writers who live in the treehouse and are supposed to be writing a book, but end up having all sorts of whacky adventures instead.
There is also a 26-Storey, 39 Storey, and 52 Storey Treehouse.