First Principles of Interaction Design (Revised & Expanded)
Bruce Tognazzini
http://asktog.com/atc/principles-of-interaction-design/
Ten Usability Heuristics for User Interface Design
Jakob Nielsen
http://www.nngroup.com/articles/ten-usability-heuristics/
The Elements of User Experience
Jesse James Garrett
http://www.jjg.net/elements/pdf/elements.pdf
Usability and UX Articles Archive
Nielsen Norman Group
http://www.nngroup.com/articles/
Navigation: You Are Here
Susan Farrell
http://www.nngroup.com/articles/navigation-you-are-here/
Basic Patterns for Mobile Navigation: A Primer
Raluca Budiu
http://www.nngroup.com/articles/mobile-navigation-patterns/
Long-Term Exposure to Flat Design: How the Trend Slowly Decreases User Efficiency
Kate Meyer
http://www.nngroup.com/articles/flat-design-long-exposure/
Page Parking: Millennials' Multi-Tab Mania
Kate Meyer and Jakob Nielsen
http://www.nngroup.com/articles/multi-tab-page-parking/
Apple Is Making Us All My Grandmother
Kara Pernice
http://www.nngroup.com/articles/apple-makes-grandmothers/
Flat Design: Its Origins, Its Problems, and Why Flat 2.0 Is Better for Users
Kate Meyer
http://www.nngroup.com/articles/flat-design/