Usability, User-friendliness and User Experience Design
Usability 101: Introduction to Usability Jakob Nielsen http://www.nngroup.com/articles/usability-101-introduction-to-usability/
International Standards for HCI and Usability UsabilityNet http://www.usabilitynet.org/tools/r_international.htm
The Definition of User Experience Jakob Nielsen and Don Norman http://www.nngroup.com/articles/definition-user-experience/
User Experience Design Peter Morville http://semanticstudios.com/user_experience_design/
The Elements of User Experience Jesse James Garrett http://www.jjg.net/elements/pdf/elements.pdf
Usability/What and Why? U.S. Department of Health and Human Services http://www.usability.gov/what-and-why/index.html?view=list
Usability/UX How To and Tools U.S. Department of Health and Human Service http://www.usability.gov/how-to-and-tools/index.html
First Principles of Interaction Design (Revised & Expanded) Bruce Tognazzini http://asktog.com/atc/principles-of-interaction-design/
Accessible Design for Users With Disabilities Jakob Nielsen http://www.nngroup.com/articles/accessible-design-for-users-with-disabilities/
Essential Components of Web Accessibility W3C https://www.w3.org/WAI/intro/components.php
Usability Guidelines for Accessible Web Design Kara Pernice and Jakob Nielsen http://media.nngroup.com/media/reports/free/Usability_Guidelines_for_Accessible_Web_Design.pdf
Stanford Guidelines for Web Credibility Stanford Persuasive Tech Lab https://credibility.stanford.edu/guidelines/
Summary of Usability Inspection Methods Jakob Nielsen http://www.nngroup.com/articles/summary-of-usability-inspection-methods/
Attractive Things Work Better Don Norman http://www.jnd.org/dn.mss/CH01.pdf