The HathiTrust Data API can be used to retrive content (page images, OCR, and in some cases whole volume packages), as well as metadata for HathiTrust volumes. There are two methods of accessing the Data API: via a Web client , requiring authentication (users who are not members of a HathiTrust partner institution must sign up for a University of Michigan "Friend" Account ), and programmatically using an access key that can be obtained at http://babel.hathitrust.org/cgi/kgs/request . Complete documentation of the API is available below. Version 2 is the most recent version.
Data API Documentation
Version 2, Revised 26 May, 2015 - Download PDF 
Data API related applications - Key Generation Service (for programmatic use) and Web client - Download PDF