I'm going to quote the whole thing, but credit is due to Bulmakau.
I am sure many will ask what the difference between books and sites is. And since we all like google scanning out sites, why should we oppose book scanning?
Well, for several reasons:
1. It is WE who like our sites scanned, and if not, we add a Robots.txt file. We can protect some of the content on our site, or all. and we easily know if its being spidered, so we can take action. How will that be with books? Robots.txt is not probably. You know what? if anything, it already exists in a way. All (most) books say have on them, in print, right in the beginning a text saying "copying of material from this book is not allowed unless permitted, prior, in writing, by the author or the publisher". I think that resembles a robots.txt file. no? And authors have little ability to "check the web logs" and see who scans their books.
2. We get something directly from it. Fair use dictates that google links to our sites directly. How will that be with books? You have to go to the shop to at least consider buying the book? Not likely. They can send traffic to Amazon maybe? But still, not a parallel (and if they do, I am sure they will collect referral fee ;))
3. Our sites operate in the internet. Books "operate" in libraries, stores. You go to a library? you can search there for a book. On the internet, you can search for sites. Not only that, but internet has shaped to be mostly a free and open medium. Books - not. Books, you have to buy, or at least subscribe to a library (paid, directly or indirectly). Different "market".
4. Most of our sites are free, and are freely accessed. Most of the sites in google are such sites. As a matter of fact, subscribed sites where their content is protected and paid for (as books are) do NOT have their content on google. And IF they have, THEY take the steps to get it into google. Books are in a sense like protected/paid sites.
A world of difference that is going to be erased very abruptly by google..
Good or bad? You decide (and also authors, publications and libraries which seems to have decided already)