Hathi (pronounced hah-tee) is the Hindi word for elephant, an animal
highly regarded for its memory, wisdom, and strength. Trust is a core
value of research libraries and one of their greatest assets. In
combination, the words convey the key benefits researchers can expect
from a first-of-its-kind shared digital repository. Emory is a member of the Hathi Trust.
More books are being born electronically or in dual formats. Emory Libraries are purchasing both print and electronic titles. Expect to find access to both in the library catalogs. Unfortunately, there are still problems of standardization with ebooks. The latest information is available in the library's e-Books guide
In addition, books previously published are being scanned into electronic format. The Emory Libraries has its own digitization program, as do most other academic libraries, but the largest efforts have been Google Books and the Hathi Trust. Both these efforts cover books in copyright as well as volumes in the public domain. Books in copyright can be identified but no free full text access is available.
Short Answer: From the Library Partnership Program begun in 2004 where Google worked with partner libraries (Univ. of Michigan and others) to digitize their collections and new publishing partners, which submit titles to Google. Users can use Google Books to identify books (full-text searching or title/author, etc). If the book is in public domain (usually pre-1923) you may have access to the full content; if under copyright, you have access to "snippets" of text (hopefully enough so you can decide to purchase the book or locate in a library). There are a significant number of books in public domain which are still "locked" in Google.
HathiTrust is a partnership of major research institutions and libraries working to ensure that the cultural record is preserved and accessible long into the future. Most of the original Hathi Trust libraries were part of the Google Library Partnership Program. The Hathi Trust libraries have digitized over 8 million volumes, about 25% of which are fully available in the public domain. There is much overlap between the Hathi Trust Digital Library and Google Books but Hathi Trust libraries do include digitized access to materials in various special collections that are not found in Google Books. Hathi Trust is also is committed to provide searching and access in ways more compatible with scholarly research. Emory University Libraries joined the Hathi Trust in November 2010.
The goals of the Hathi Trust, to preserve and protect the cultural record, are different from those of Google Books. There is also some difference in content, but for the user, there is much overlap between the two. The following article suggests ways to use both for your research: Improve your use of Google Books with Mirlyn and Hathi Trust by Natalie Houston, Chronicle of Higher Education, Oct. 12, 2010.