I've used Google Books' scans many times, but I can honestly say it has never discouraged me from actually buying a book. In a few cases, in fact, it's been the opposite — having seen something interesting, I've purchased a copy of the book.
Google’s Book-Scanning Project Is Legal, U.S. Appeals Court Says
A group of authors had sued over Google’s effort to compile a digital library, arguing the project was illegal. An appeals court found the program complies with copyright law.