A post on the Google books blog reveals the company's method behind the counting. Google first defined what they considered to be a 'book' given the number of articles, works and other forms of literature in print. Duplicates were removed by using metadata from various libraries and catalogues to separate nearly a billion entries down to 600 million unique records. This had to be further reduced using an algorithm that removed any books which were the same but had different titles. Google is still working on the algorithm's accuracy, which is gives varying results.