Google has added more public-domain books to its Google Print service.

The books come from libraries at the University of Michigan, Harvard University, Stanford University and the New York Public Library and can be viewed in their entirety in the Google database.

Previously, Google Print users could find public-domain books and view their entire contents but Google calls Thursday's announcement the first significant addition of public-domain books. Google isn't revealing how many new books have been added but said that for example, US Civil War history books, government documents and works by Henry James are now available.

Public-domain books either were never covered by copyright or are no longer protected by copyright.

The Google Print program has been criticised by authors because Google includes books in the database that are still protected by copyright. Users can only view snippets of such books, however.

The Authors Guild and the Association of American Publishers have both filed lawsuits against Google, charging the company with copyright infringement for scanning books without author permission. Google allows authors to opt out of the program but historically content users have been required to proactively seek permission to use copyright content. Google maintains that because it will only display snippets of books, the Google Print program constitutes fair use and thus complies with copyright laws.