Don't Make Me Think, Revisited: A Common Sense Approach to Web Usability, 3rd Edition by Steve Krug

Very interesting treatment of designing web sites for usability and usefulness. It's written in a plain and understandable way to see why some web sites are just really hard to use. I really appreciated the discussions about usability for visually impaired and how a lot of times this is used the wrong way. Much of the navigation on many web sites is really geared towards desktop browsers and needs a lot of rethinking for mobile. Since the mobile environment doesn't have events like hover many "easy to use" features on the web are not usable at all.