The Design of everyday things [1] (Found on Coding Horror's recommended reading [2]) was crucial in understanding how people intuitively interact with the world around them - including your web application - based on the visual cue's you provide.

While this article provides some metaphorical fish - I found the Design of everyday things helps you become a fisherman.

EDIT: Swapped the order of references.

[1] http://www.amazon.com/Design-Everyday-Things-Revised-Expande...

[2] http://blog.codinghorror.com/recommended-reading-for-develop...