Don’t Make Me Think, Revisited: A Common Sense Approach to Web Usability (3rd Edition) by Steve Krug, 2014: clear easy to read content about what makes people leave webpages and how to do relevant usability testing
Designed for Use: Create Usable Interfaces for Applications and the Web by Lukas Mathis, 2011: in depth explanations and examples on how to build a user-friendly interface during the design process
W3C Web Accessibility Initiative website offers strategies, guidelines, and resources to help make the Web accessible to people with disabilities