Double integral of modulus of difference reference

Question

I am looking for some references about integrals of the form

$$ \int_\Omega \int_\Omega |f(x) - f(y)| dx dy.$$

I've only seen something like this for the definition of fractional Sobolev spaces, but I would like to know something about "integer" Sobolev spaces and more importantly without anything at the denominator. Any reference?