User-Centered Web Design , by John Cato, Addison-Wesley, 2001.
"Never forget who the audience is" is the refrain in this very grounded approach to how people use Web sites and systems.
Designing Web Usability: The Practice of Simplicity , by Jakob Nielsen, New Riders Publishing, 2000.
Very XP. Nielsen's straightforward and lightweight suggestions help sites communicate with their audience without gimmicks or fanfare.