Usability Engineering by JakobNielsen (ISBN: 0125184069 )
This is one of the standard texts on Usability issues. A lot of user interface books tend to be very loose on testing and processes. This book provides a good balance against books like AboutFace and TogOnInterface.
-- ToddCoram
See also Jakob's website at http://www.useit.com/ -- BrianEwins
"Alertbox: Current Issues in Web Usability," a bi-weekly column by JakobNielsen: http://www.useit.com/alertbox/