A colleague recently recommended taking a look at
Steve Krug's
Rocket Surgery Made Easy, as although it is primarily aimed at individuals/companies who wish to conduct their own usability testing, some of the insights and ideas are useful from the perspective of a consultant.
Krug gives clear and succinct instructions for running tests, as well as guidelines for identifying and fixing issues.
There were some really good points to consider, particularly when it comes to considering how to making findings actionable. There were also a few good pointers in regards to a few of the basics which can sometimes get lost/forgotten.
Will need to have another look at
Don't Make Me Think, as well as some of the suggested reading.