"effectively read (meaning you can read the bit you're looking for, not the whole thing) for free"
Well if there are only three pages worth reading in the entire book and they happen to come one after the other, then perhaps the book wasn't all that good in the first place.
Alternatively, perhaps you actually *would* be tempted to buy the rest of the book if only it wasn't so expensive. In that case, is it not reasonably to argue that Google are making it possible to sell the book at a lower price by making it easier for readers to find that, yes, this probably is a book they are interested in.
I've certainly bought books after reading tasters and snippets online. I've also browsed books in a real bookshop. I don't see much difference. It certainly seems at least a likely to reward authors as putting books in public libraries and paying authors some fraction of a penny each time their book is borrowed. (I've bought books after borrowing them from the library, too, which presumably paid the author and publisher an order of magnitude or two more than simply having the book in the library.)
Lastly, I think the ability of almost anyone to self-publish on the interwebs probably does as much to kill organised scholarly textbook writing. There's some great stuff out there on way out subjects, written by people who apparently do it for the pure joy of getting their *own* heads round the subject matter. The world is changing. Google is actually quite late to the party and is only picking at the left-overs. In this case, the left-overs are "anything from the past five centuries that still seems interesting, which to be honest is less than you might think".