I haven't done any formal usability testing yet, but it will be a focal point of my final year project at university, so I've been looking for books that explain the process. Measuring the User Experience initially seems like a textbook full of boring statistics, but on further inspection the information inside is very practical. Tullis and Albert's book sets out to cover the whole process of doing a usability study, from planning and collecting data all the way through to analysing and presenting it.
If Steve Krug's Don't Make Me Think! is a first step into the world of usability, then Measuring the User Experience is great choice for where to advance your other metaphorical foot to next. For someone like me, who is familiar with many of the concepts but has never had a chance to apply them in a formal study, Tullis and Albert's book is highly recommended.