I'm trying to better understand User Experience and Interaction Design, and I feel like this is a fundamental text on it - http://asktog.com/atc/principles-of-interaction-design/. I haven't formally studied HCI outside of philosophy classes that touched on the Turing Test and cognition, but, I have always been deeply inquisitive about how the role of the user fits into site design.
Some of my favorite kernels of truth are:
1) "Principle: User test the visual design as thoroughly as the behavioral design.
User test after aesthetic changes have been made, benchmarking, where applicable, the new design against the old. Ensure that learnability, satisfaction, and productivity have been improved or at least have stayed the same. If not, newly-added aesthetics that are causing a problem need to be rethought."
2) "Principle: The computer, interface, and task environment all “belong” to the user, but user-autonomy doesn’t mean we abandon rules.
"Adults feel most comfortable in an environment that is neither confining nor infinite, an environment explorable, but not hazardous."
3) "Principle: Any attempt to hide complexity will serve to increase it.
'Functional software does not have to look like a tractor; it can look like a Porsche. It cannot, however, look like a Porsche that’s missing its steering wheel, brake, and accelerator pedal. "
If you haven't ever before or you just want a refresher, read through the Principles of Interaction Design -- it seems more appropriate now than ever before. I think this text probably pairs well with The Design of Everyday Things.