William's Picks
The 13-Storey Treehouse, The 26-Storey Treehouse and The 39-Storey Treehouse by Andy Griffiths
This series is about 2 kids called Andy and Terry and In each book they have a little adventure.

In book 1 they create an evil sea monster that tries to eat them.

In book 2 their treehouse gets taken over by pirates.

In book 3 they create a machine to make books but it locks them out of their treehouse.

I liked this series and would recommend it.