In terms of the usability process and conducting tests:
Interaction Design by Jenny Preece, Yvonne Rogers & Helen Sharp.
A practical guide to usability testing by Joseph Dumas and Janice Reddish.
In terms of the general concept of usability and interaction design:
The Design of Everyday Things by Donald Norman
Designing Interactions by Bill Moggridge
Papers:
Experience Prototyping: Suri & Buchenau (IDEO)
Participatory Design: Exploring ambient displays for social awareness: Williams, Farnham & Counts
