Usability Testing Essentials: Ready, Set...Test! by Carol Barnum. A few folks have recommended this book to me under the incomplete title "Ready, Set, Test" so I had a hard time tracking it down. From the description on Amazon:
Usability Testing Essentials presents a practical, step-by-step approach to learning the entire process of planning and conducting a usability test. It also explains how to analyze and apply the results and what to do when confronted with budgetary and time restrictions. This is the ideal book for anyone involved in usability or user-centered design-from students to seasoned professionals.
"Current Issues of Usability Characteristics and Usability Testing" by Ayobami, Hector, Hammed. It's a short read (five pages, including the references) but provides a good summary of the issues. From the abstract:
A well given design’s attention to Learnability, Contextuality and Operationability as Usability characteristics issues of software, and information systems generally is an evolving and challenging area in the field of Information systems, and Human Computer Interaction as an area of expertise. The procedure of testing and evaluating usability as quality characteristics in information systems has equally attracted researchers and professionals more in the recent times due to the developers’ attempt to meet the diverse users’ needs in the development of usable information systems. Hence, the paper aims to bring to the limelight; these current issues of usability and usability testing, the trend of the challenges inherent in the usability testing and evaluation process, then suggest the necessary attention that must be provided by subsequent researches in the bid to solve these lingering design research problems.
The article quickly identifies the key issues of usability: Learnability, Contextuality and Operationability. If your design can master those qualities, you should be okay. This is echoed somewhat through my findings in open source software and usability, where open source software that exhibits successful usability shares these characteristics: