Prelinger Library – Online Collection of Public Domain Materials in Key Subject Areas
http://www.archive.org/details/prelinger_library

The Prelinger Library is a private research library open to the public, located in downtown San Francisco. Its collections encompass some 50,000 books, periodical volumes and printed ephemera. They are currently curating an online collection of public domain materials in key subject areas. This has been added to Reference Resources Subject Tracer™ Information Blog.