ITS Status report (includes architecture info) from 1972
ftp://publications.ai.mit.edu/ai-publications/0-499/AIM-238.ps

ITS reference manual from 1969:
ftp://publications.ai.mit.edu/ai-publications/0-499/AIM-161A.ps

Midas macro assembler documentation from 1965:
ftp://publications.ai.mit.edu/ai-publications/0-499/AIM-090.ps

DDT debugger ("system apex procedure" i.e. "shell") from 1968
ftp://publications.ai.mit.edu/ai-publications/0-499/AIM-147.ps

Emacs text editor (original ITS version)
ftp://publications.ai.mit.edu/ai-publications/500-999/AIM-519A.ps

These files may be converted to PDF or gzipped to save space:
$ ps2pdf13 AIM-238.ps AIM-238.pdf