Since March, 2012, the Office of the Historian at the U.S. Dept. of State has been releasing digitized volumes of the Foreign Relations of the United States series and making them available online.
103 volumes are currently online (free) and available in Mobi and EPUB formats. More volumes will be made available in the future.
Today, the Historians Office released their first API, the Ebook Catalog API.
From the US DOS Developer Page:
Use this API to embed our live, up-to-date, searchable, and browsable ebook catalog in your application and to locate download links to the ebooks and their cover images.
Our ebook catalog API uses the Open Publication Distribution System (OPDS), an Atom XML-based standard for encoding ebook catalogs.
See Also: EBook Catalog API Documentation
See Also: Support for API (via Github)