If the book is out of copyright, or the publisher has given Google Books permission, you'll be able to see a preview of the book, and in some cases the entire text. If it's in the public domain, you're free to download a PDF copy.
Source: About Google Books
When you find a book that's still under copyright, you'll see only a small portion of the book at a time–either the Snippet View or the Sample Pages View–plus links to places where you can buy or borrow it.
Source: Google Books - Perspectives - Fact & Fiction