Google Books Advanced Search allows you to restrict it to only full-text books if you want. You can also narrow down your search by publication date and language.
You can use the regular Google tricks:
For example, +"new york" -subway +train* railroad
Google Books contains both public domain and copyright protected content.
Public domain books are available in full-text to be searched and read.
For copyright protected books, you can search the full-text and preview the content. Google also shows you online stores where you can buy a copy (new or used, in print or e-book when available.)
In the future, for copyright protected books that are out of print, you will be able to purchase permanent full-text access to an individual book. Check back here for more information as it becomes available.