The 39-Storey Treehouse by Andy Griffiths and Terry Denton

Any and Terry live in the world's coolest treehouse! They've been doing some building work and now their treehouse has an active volcano for toasting marshmellows, jet-propelled chairs to escape the Trunkinator and the world's scariest rollercoaster...unless Professor Stupido, the world's greatest univentor, uninvest it all!
 
Suitable for ages 9-11
 
Collect the series