They... don't impact the environment. The trees they use were planted for the sole purpose of being harvested. And new trees are planted for every tree that's harvested.

~~If anything, they're a net positive for the environment, since they're sequestering carbon into books.~~

EDIT: I'll acquiesce that they may not be a net positive, since there are carbon costs associated with the entire process. But, using purpose-grown trees is hard to consider as a net negative.

And, if we're talking about the entire process, e-books are not carbon-zero products either.