The 169-Storey Treehouse by Andy Griffiths and Terry Denton
 
The last book in the Treehouse series!
 
It has everything you could wish for, including: an electric pony stable, a NOISY level, a weather dome, an 100% edible gingerbread house, a potato-powered translation transmitter that allows you to talk to everything everywhere all at the same time. There's also a hall of funhouse mirrors, which is the perfect place to hide from Mr Bunkoff who is trying to catch Andy, Terry and Jill in order to send them to school.

But Anti-Andy, Terrible Terry and Junkyard Jill, their trouble-making opposites, are trapped in one of the mirrors. Can our heroes escape school, defeat their doppelgangers AND meet their book deadline? Well, what are you waiting for? Come on up!