I was reading this book on anti-gravity

I was reading this book on anti-gravity.
I couldn't put it down!

This joke is a weird one. The humor lies in the fact that anti-gravity would make something not fall, but instead float. If you were holding a book that was repelled by the earth, when you let go of it it would fall up toward the ceiling (or if you were outside, the sky).

The humor lies in the fact that this anti-gravity subject matter book is also apparently an actual anti-gravity book as well, and that was unexpected.