Recently I saw a list created by Paul Olyslager, of 15 free ebooks about UX and interface design, including some best selling books which are definitely worth reading.

Search User Interfaces by Marti A. Hearst (2009)

If you want to design innovative search user interfaces, you need this book close at hand at all times.
http://searchuserinterfaces.com/book

Web Style Guide by Patrick J. Lynch and Sarah Horton (1999)

A style guide for the interface with real long-run value, showing us deep principles of design rather than simply fashion and technology.
http://www.webstyleguide.com

Designing Interfaces – Patterns for Effective Interaction Design by Jenifer Tidwell (2006)

Anyone who’s serious about designing interfaces should have this book on their shelf for reference.
http://designinginterfaces.com/patterns

Designing Mobile Interfaces by Steven Hoober and Eric Berkman (2011)

This practical guide teaches you the core principles for designing effective mobile user interfaces, and helps you get started by providing more than 40 proven UI patterns for mobile websites and applications.
http://4ourth.com/wiki

Mental Models in Human-Computer Interaction by John M. Carroll and Judith Reitman Olson (1987)

Research Issues About What the User of Software knows.
http://books.nap.edu

Just Ask: Integrating Accessibility Throughout Design by Shawn L. Henry (2007)

A focus on accessibility during the entire user-centered design process.
http://www.uiaccess.com

Converting The Believers by Usereffect (2008)

Turn believers into buyers and maximize your website’s ROI. Learn how to use analytics, usability and testing to systematically improve online sales.
http://www.usereffect.com/ebook

Task-Centered User Interface Design by Clayton Lewis and John Rieman (1994)

This report is based on 8 series of usability studies with users in 4 countries reporting how they actually used a broad variety of websites and apps on a range of mobile devices, including touch phones and smartphones from many vendors.
http://hcibib.org/tcuid/

User Centered Design – The Fable of the User-Centered Designer by David Travis (2009)

After reading this 40-page fable, you’ll understand the framework of user-centred design and know how to apply it to your own design project.
http://www.userfocus.co.uk

234 Tips and Tricks for Recruiting Users as Participants in Usability Studies by Deborah Hinderer Sova and Jacob Nielsen (2003)

This report tells you how to set up and manage a recruiting program, how to get the right users for specific tests, and how to deal with the users you have recruited.
http://www.nngroup.com

Introduction to Good Usability by Peterpixel (2008)

A collection of Peter’s posts about design guidelines in PDF format.
http://www.peterpixel.nl

UX Storytellers – Connecting the dots by 42 UX masterminds (latest update 2011)

42 UX masterminds tell personal stories of their exciting lives as User Experience professionals.
http://uxstorytellers.blogspot.com

A Hitchhikers Guide to the Obvious: Web Usability 101 by Squiz (2009)

The design best practices for usability and how they can positively impact your bottom line. The paper also gives concrete guidelines on how to test for usability on a shoestring budget.
http://www.squiz.co.uk

Designing the Mobile User Experience by Barbara Ballard (2007)

Gain the knowledge and tools to deliver compelling mobile phone applications.
http://freepdfbookdownload.com

Human Computer Interaction Lecture Notes by Dr. Keith Andrews (2011)

These lecture notes have evolved since he first started teaching human-computer interaction at Graz University of Technology in 1993 and have benefitted from his teaching courses on user interface design at FH Technikum Kärnten in Villach, web usability and advanced user interfaces at FH Joanneum.
http://www.pdfebookshub.com

Bonus: Research Based Web Design & Usability Guidelines by Usability.gov

Throughout your Web design or redesign project, you should take advantage of what is already known about best practices for each step of the process. The Research-Based Web Design and Usability Guidelines, compiled through an extensive process of research and review, bring you those best practices.
http://www.usability.gov