The truly worldwide reach of the Web has brought with it a new realisation among computer scientists and industry professionals of the enormous importance of usability and user interface design. In the last ten years, much has become understood about what works in user interfaces from a usability perspective, and what does not.

The following are free-to-read books on user experience, available online:

Human Computer Interaction

• Mental Models in Human-Computer Interaction: Research Issues About What the User of Software Knows by John M. Carroll and Judith Reitman Olson
• HCI Models, Theories and Frameworks: Toward a Multi-disciplinary Science by John M. Carroll

User Experience / Interaction Design

• Search User Interfaces by Marti A. Hearst
• Designing Interfaces – Patterns for Effective Interaction Design by Jenifer Tidwell
• Thoughts on Interaction Design by Jon Kolko. with contributions from Ellen Beldner, Uday Gajendar, Chris Connors and Justin Petro
• The Fable of the User-Centered Designer by David Travis

Web Accessibility

• Just Ask: Integrating Accessibility Throughout Design by Shawn Henry
• Building Accessible Websites by Joe Clarke
• Dive Into Accessibility: 30 Days to a More Accessible Website by Mark Pilgrim

If there are any more you think need or can be added to the list, please leave a comment.