If a book is out of copyright, or the publisher has given Google 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 Internet Archive and Open Library offers over 8,000,000 fully accessible public domain eBooks. This includes a special modern collection of over 500,000 eBooks for users with print disabilities, and a very interesting curated, modern collection for the world at large. You can browse, read and borrow fascinating contemporary materials at OpenLibrary.org.