The HathiTrust Digital Library started with the collection of the University of Michigan Library, which was digitized by the Google Books Project.  Since then the Digital Library has grown to include collections digitized from other partner libraries and research institutions as well as collections from other digital projects like the Internet Archive.

Full-text access and downloading is available for those items in the public domain, including:

• US federal government documents
• Works published before 1923
• Works still protected by copyright, but made available to HathiTrust with the permission of the copyright holder

The number of works in the HathiTrust Digital Library is large and ever-increasing.  Currently digitized as of August 2018:

• 16,468,932 total volumes
• 8,016,383 book titles
• 442,413 serial titles
• 5,764,126,200 pages
• 738 terabytes
• 195 miles
• 13,381 tons
• 6,226,134 volumes(~38% of total) in the public domain

Comparison of the Google Books Project with HathiTrust:

Thanks to Barbara DeFelice and William Fontaine of Dartmouth College for this chart.

In addition to their informational value, many items in HathiTrust have interest as historical, cultural, or archival objects.  This Civil War era diary of Lucius L. Shattuck stopped a bullet and the damage can be traced deep into the book.  The original can be found in the Bentley Historical Library at the University of Michigan.