While Google Books has digitised millions of books all over the world with the help of university libraries, not all of those digitised books are freely available on the website. Books that are still in copyright cannot be consulted in full-text.
Sometimes, however, Google has not assessed the copyright correctly and the book is not publicly available, although Google has scanned it and it is out-of-copyright. That is the case with all books published before 1900 and some books published between 1900 and 1930.
When you know that Google Books has a scan of the book available and you believe that it is in the public domain, you can ask Google to re-evaluate the copyright situation of that publication. They will do so in a couple of days or a couple of weeks the latest.
In order to request a copyright re-evaluation of a Google Book scan follow these steps: