This is a short and sweet introduction to usability, targeted specifically at web design, but generally applicable to broad areas of interface design. I picked it up because it seems to be commonly cited as a classic of the field. It's a well-designed, straightforward book that hammers home the point that there's nothing complicated about usability and user interface design—it really is just refined common sense.

Krug's three laws of usability are well-taken, but the most useful parts for me were the chapters on informal usability testing and accessibility. For the latter, there appear to be some good references for further reading. This is a very quick read, and there isn't a lot of depth, but it seems like a decent overview.