Thanks to The UX Bookmark for this great list of free online user experience books!
1. Search User Interfaces– by Marti A. Hearst
2. Web Style Guide– by Patrick J. Lynch and Sarah Horton
3. Designing Interfaces– by Jenifer Tidwell (Most of the book is available online, not all of it, as pointed out by Amanda)
4. Designing Mobile Interfaces– by Steven Hoober and Eric Berkman
5. The Psychology of Menu Selection: Designing Cognitive Control at the Human/Computer Interface- by Kent L. Norman
6. Mental Models in Human-Computer Interaction: Research Issues About What the User of Software Knows– by John M. Carroll and Judith Reitman Olson (editors)
7. Just Ask: Integrating Accessibility Throughout Design– by Shawn Henry
8. Building accessible websites– by Joe Clark
9. Introduction to Metadata – by Tony Gill, Anne J. Gilliland, Maureen Whalen, and Mary S. Woodley
10. Introduction to Controlled Vocabularies: Terminology for Art, Architecture, and Other Cultural Works– by Patricia Harpring
11. Categories for the Description of Works of Art– by Murtha Baca and Patricia Harpring
Lisa Duddington 