Advertisements contain the only truths to be relied on in a newspaper.
    —Mark Twain

Jason Kottke talks about advertising in books as a way to lower the price of books. He also makes the claim that:

I believe the world is a better place without advertising absolutely everywhere. But if advertising makes books more affordable — and in some cases absolutely free — and therefore accessible to more people, it’s hard to argue that it wouldn’t be a good idea.

Dare I say, go to the library for free? (Unless you live in Seattle, of course)

Going one ludicrous step further and following the Hollywood paradigm, there could be paid product placement in novels:

Dickens:
“It was the best of times, it was the worst of times. You can have even better times with Fritos® brand corn chips!”

Melville:
“There she blows!- there she blows! A hump as graceful as the new Volkswagen Beetle®! It is Moby-Dick! Drivers wanted.®”

Shakespeare:
“Sleep seldom visits sorrow; when it doth, it is a comforter. Sorrow getting the best of you? Try Paxil®, because your life is waiting!”

Emily Dickinson:
“Because I could not stop for death, He kindly stopped for me; The carriage held but just ourselves and immortality and my Nokia® 6800 cell phone.”