A brick.
If this one already made an appearance on here, I’m sorry. Just got reminded of it while thinking about idioms for hefty books, in english I couldn’t find any. In my first language we would say “it’s a brick”
Alt version of this joke:
What's red and good for a cop's teeth?
Correct! A brick!
That is an anti-joke, not a dad joke. But it is funny and tangential. I tell it frequently. Another:
What’s blue and tastes like red paint?
Blue paint.