Fitting in with Google’s overarching mission “to organize the world's information and make it universally accessible and useful” (http://www.google.com/corporate/), Google Books is a large scale digitization project that includes current material that publishers have agreed to supply as well as the more controversial older content held by major research libraries in the United States that has been digitized by Google. The outcome of the Google Book Settlement will ultimately define the future of Google Books.
A subsidiary of the Internet Archive, the goal of Open Library “is to list every book -- whether in-print or out-of-print, available at a bookstore or a library, scanned or typed in as text.” (http://openlibrary.org/help/faq). While Open Library does have access to public domain books via the Internet Archive, their primary goal is to serve as a catalog, with a secondary goal of getting the user to the text – either thru scanned books or from bookstores.
The World Digital Library “makes available on the Internet, free of charge and in multilingual format, significant primary materials from countries and cultures around the world, with principal objectives that include: promoting international and intercultural understanding; expanding the volume and variety of cultural content on the Internet; providing resources for educators, scholars, and general audiences; and building capacity in partner institutions to narrow the digital divide within and between countries. (http://www.wdl.org/en/about/)
There is obviously an overlap of full text content among Google Books, Open Library, and HathiTrust, since the HathiTrust founding partners participated in the Google Books Project or the Internet Archive digitization initiatives. However, beyond HathiTrust’s fundamental goal to preserve the human record, there are other differences between HathiTrust and Google Books: