Computing Sciences Technical Reports
Selected (by virtue of easy electronic access) Bell Labs papers, many of which deal with Unix or Unix utilies. Eg, #54, "Nroff/Troff User's Manual" by Ossanna with Kernighan revisions and #123 "C Traps and Pitfalls" by Koenig, later adapted into a book of the same name. Many of these are gziped postscript files.
dead link: http://cm.bell-labs.com/cm/cs/cstr.html