In creating a great website or app, you have a diverse set of guidelines and considerations to deal with. Despite the wide variety of issues, opinions, and platforms, there is one underlying constant you can depend on (we hope): the user is a human being.
If you'd like to understand how human perception should inform your design, I highly recommend Jeff Johnson's new book, Designing with the Mind in Mind.
The book is a fascinating blend of cognitive science and user interface design. It doesn't present UI principles in the usual sense, but rather illustrates principles of human perception, and how to use that knowledge to improve your designs.
Visual perception is an amazing thing. It's all too easy to think of our eyes as a sort of digital camera, with an array of pixels at high density. But it's really something very different, with a tiny spot in the center with very high resolution, and then quite low-resolution sensors for areas even slightly off-center. This fact, combined with the ways visual perception is influenced by memory and expectations, has enormous implications for user interface design, and this book explains them lucidly.