The books on this list have done more than sell a lot of copies (some have only sold a modest amount). They have been influential in providing material that has helped establish the usability profession.

1. A Practical Guide to Usability Testing by Joe Dumas & Ginny. Redish (1993)
One of three books on the mechanics of usability testing. It also includes a detailed section on how usability testing fits into an organizations development process.
2. Handbook of Usability Testing by Jeffrey Rubin (1994)
Focuses on the important details and mechanics of conducting a usability test. It is now in its second edition with some new material from Dana Chisnell, and Jared Spool.
3. Usability Engineering by Jakob Nielsen (1993)
Includes detail on usability testing and heuristic evaluations. If you’re used to Nielsen’s more recent work with the emphasis on qualitative insights you may be surprised to see the amount of quantitative research and data supporting the methods.
4. Cost Justifying Usability by Randolph Bias & Deborah Mayhew (1994)
A perennial issue for anyone in usability is justifying your work, your budget and your results. You may not always convince others, or even yourself that usability provides a measurable ROI but this is the place to start when you need to. Also in its second edition with a lot more material.
5. The Psychology of Human Computer Interaction Stuart Card, Thomas Moran & Allen Newell (1983)
Don’t try and read this in one sitting. This is the text that explains Keystroke Level Modelling (KLM) and GOMS (Goals, Operators, Methods & Selection Rules). It is still the best resource for understanding how to predict skilled users’ time[pdf] (an under-utilized tool in my opinion).

Any short list is going to leave off a lot of other important books. Here are just a few that didn’t make my top five.