Design by People for People is a terrific little book full of useful gems for people faced with the questions that arise from regular usability testing: how many participants, when to intervene in a usability test, effective think-aloud methods. However this book is written in such a straightforward and engaging manner, it’s far less painful than digging through academic screeds.
It also looks at consulting issues (not to be missed — Rubin’s essay on Authentic Consulting) and even experience design.
It’s not a book for you if you have never done usability testing before.. Rubinï¿½s Handbook is better for that. But if you want to refine your skills, definitely check it out.