ELEVATOR TO NOWHERE (Give Yourself Goosebumps #34) by R.L. Stine

GOING UP...AND OUT OF THIS WORLD!

Meet Uncle Darius. He's an inventor — and your ticket to an A on your science project. The trouble is he's a little wacky. Okay a lot wacky! Right now he's testing out his Transuniversal Transvator. It looks like an elevator. But it takes passengers to other universes! Uncle Darius says he'll prove it by bringing back a special souvenir — a shrunken head.

But when Uncle Darius returns, something is wrong. How wrong? He wants to shrink your head! And your only means of escape is battling this headhunter on Earth — or battling the unknown in another universe. Hurry, the doors are closing!