Don't make me think! Common sense web usability
I wrote this post a while back. The content can still be relevant but the information I've linked to may not be available.
Don't make me think. It's a great title for a book – and the contents are good as well. Some simple but fundamental guidelines about website usability written in an easy-to-read style. I certainly found it very 'user-friendly'.
The second edition features three new chapters including a chapter on Accessibility and Cascading Style Sheets.