For those interested in studying primary materials (source code, documentation, etc.) from the Alto system built at Xerox PARC in the 1970s, the Computer History Museum has released an archive with snapshots of the Alto file servers at PARC taken at various times from the mid 1970s to the mid 1980s. 

This @CHM blog post introduces the archive:

A “walk-through” of the archive is here: http://xeroxalto.computerhistory.org/xerox_alto_file_system_archive.html

The archive is here: http://xeroxalto.computerhistory.org/

More detail about the archive (provenance, naming conventions, file types, etc.) is available here: http://xeroxalto.computerhistory.org/xerox_alto_file_system_archive.html/#Archive .

Paul McJones