Moderating Usability Tests: Principles and Practices for Interacting (Interactive Technologies) 1st edition by Dumas, Joseph S., Loring, Beth A. (2008) Paperback
This is very good basic book of how to conduct and moderate usability tests. I would recommended this book for usability students and/or usability specialist that are about to moderate their first session. For experienced usability specialists there is not really anything new.