Even if a text was available online I believe that most people would go
down to the bookstore and buy a copy to read if they wanted to read it.
*Assuming* they didn't have to buy it twice (once online and once hardcopy)!
Sales of certain types of books (mainly reference books) would probably
be affected by publication online but the vast majority would not. Even
so, I seriously doubt that it would make very much difference except in
cases where the printed material is overpriced creating a demand for
black-market goods (and in that case the free market will probably have
made a wise choice :-).
But you are right, if someone started scanning in texts and posting them
around the net it would drive publishers nuts :-)