The Best of Both Worlds: Combining Usability Testing and Documentation Projects 
Describes two cases in which usability testing and documentation projects were performed in conjunction with one other. It describes how usability testing affected the design and content of the documentation and how follow-on usability studies added significant new data not revealed in the initial tests.
Kantner, Laurie, Stephanie L. Rosenbaum and Connie Leas. Tec-Ed, Inc. (1997). Articles>Documentation>Usability