If the book is out of copyright, or the publisher has given us 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.
The Library Project makes it possible to search through millions of books via Google, including books that are rare, out of print, or generally unavailable outside of a library.
If a book from the Library Project is determined to be in the public domain, Google will make it fully available to the public. If not, you'll still be able to search the content of the book, but Google Books will only display a selection of the text (highlighting where your search terms appear in the text).
How books are displayed in Google Books: