You may not be able to get the full text of the copyrighted work, but Google can and has. Google are profiting from an unauthorised copy made of a copyrighted work. If google are allowed to do it, why can't I? I only want to make one copy of each book from the library. I don't intended to sell that unauthorised copy to anyone, heck I don't even intend to let anyone else see even snippets of it. What's the difference? Why are Google allowed to make copies for their own purposes but I am not? Is it because they are a rich company who can afford lawyers to override copyright laws?
You can. Provided that it's a fair use. Any use can be fair, but not every use is; it depends on the circumstances. There's a four factor analysis that's usually done in order to determine whether a use is fair or not, though sometimes one factor can outweigh the others, and it's a good idea not to lose sight of the big picture.
If you copy books for your own use as a substitute for buying them, my guess is that this is not a fair use. OTOH what Google does is copy books not in order to use them itself, but to aid others in searching them, and when it presents information unauthorizedly copied from copyrighted books to users, it only provides a tiny amount, not the entire thing. This makes the search a useful adjunct to the books, but not a substitute.