Rick Anderson has published "5 Million Public Domain Ebooks in HathiTrust: What Does This Mean?" in The Scholarly Kitchen.
Here's an excerpt:
A week or so ago, a monumental thing happened: the number of public-domain books in the HathiTrust digital repository topped 5 million. And since no one (including HathiTrust, so far) seems to be making a very big deal about this, it seems like a good moment both to recap the achievements of HathiTrust and to consider a few of its implications for the future of reading and scholarship.
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