Correction, I failed to include an important web site to my list.
For case studies check out comparing UNIX to MPE/iX:
http://aics-research.com
www.adager.com
http://www.aberdeen.com - did a study of the HP 3000 and user base
http://www.hp.com/csy
Ask Ron Seybold of the HP NewsWire
Search back issues of INTEREX publications - www.interex.org