By Maggie Shiels
Technology reporter, BBC News, Silicon Valley
Up to five million books are known as "orphan" books
Google has defended its online book deal amid reports it is being reviewed by the US justice department.
An investigation is expected to examine whether or not the book search agreement with authors and publishers violates anti-trust laws.
Google reached a settlement in October to create a $125m (£85m) fund to pay authors to have their work digitised.
The deal still needs court approval and this week the deadline was extended to September for others to oppose it.
Now that the justice department has become involved, Google has taken to the blogosphere with a post explaining the benefits of putting millions and millions of books online.
"As the discussion continues, it's important to understand what readers stand to gain," wrote Adam Smith, the director of product management for Google Book Search.
In his post, Mr Smith outlined a number of scenarios where readers could benefit. These include being able to access out-of-print, or so-called "orphaned" books where the authors cannot be found, as well as being able to get hold of more books in foreign languages.
For authors, Google also stressed the positives.
"The settlement won't just expand access to out-of-print books, either," said Mr Smith.
"Because authors and publishers will have the ability to let users preview and purchase their in-print books through Google Book Search, readers will have even more options for accessing in-print books than they have today."
The Google settlement was reached following earlier legal action by the Authors Guild and the Association of American Publishers.
That suit claimed that Google's practice of scanning copyrighted books from libraries for use in its Book Search service was a violation of copyrights.
Google's service will allow users to download PDF files of classic books
Both organisations refused to comment on the story.
The Department of Justice also declined to give a statement. Its involvement does not necessarily mean it will attempt to scuttle the deal, which critics have said would unfairly give Google an exclusive licence to profit from millions of books.
Some librarians have said they fear that with no competition Google would be free to raise prices to access the collection.
The Consumer Watchdog told the BBC it was one of a number of groups involved in calling on the Department of Justice to act.
"We felt the deal set up an unfair monopolistic situation for Google," explained Consumer Watchdog advocate John Simpson.
"We do need to have the world's books digitised but I think there are very big concerns if one internet giant is able to dominate the digital market. We want a level playing field here," Mr Simpson said.
The deal currently only applies in the US, but Google's Adam Smith said: "We believe that this will constitute an unprecedented test bed for the development of similar services around the world."