1979 conference paper by Dennis Ritchie. "Concentrates on the evolution of the file system, the process-control mechanism, and the idea of pipelined commands. Some attention is paid to social conditions during the development of the system." http://cm.bell-labs.com/cm/cs/who/dmr/hist.html
The full source archives of the University of California at Berkeley's Computer Systems Research Group, for sale on a 4-CD set. http://www.mckusick.com/csrg/
Built to foster the "the preservation and maintenance of historical and non-mainstream UNIX systems." Mailing list and archives. Includes early Unix source code, from 1973 through the 1990s. http://www.tuhs.org/