Free to read online books on Interaction Design
Search User Interfaces- by Marti A. Hearst
Web Style Guide- by Patrick J. Lynch and Sarah Horton
Designing Interfaces- by Jenifer Tidwell (Most of the book is available online, not all of it, as pointed out by Amanda)
Designing Mobile Interfaces- by Steven Hoober and Eric Berkman
Thoughts on Interaction Design- by [...]