Great introduction to usability from Steve Krug, which also gives me an opportunity to link to my article on usability testing.
Via 37signals.
If you liked this, you'll love my book. It's called Designed for Use: Create Usable Interfaces for Applications and the Web. In it, I cover the whole design process, from user research and sketching to usability tests and A/B testing. But I don't just explain techniques, I also talk about concepts like discoverability, when and how to use animations, what we can learn from video games, and much more.