Usability Resources
What is Usability?
Usability is the measure of the quality of a user's experience when interacting with a system like a web site. Usability of sites is measured by testing a range of factors including how satisfied users are with your site, how easy it is to navigate and the efficiency of your site
To find out more about usability check out the resources supplied by the Usability Professionals Association.
Books
If you want to buy one book on usability buy Steve Krug's Don't Make Me Think: A Common Sense Guide to Web Design. Krug teaches you how to improve web design while retaining a sense of humour. He has easy to use guides to user testing. Go to his website www.sensible.com for more information.
If you buy two books the second should be Designing Web Usability from Jakob Nielsen. The online home of Jakob Nielsen also offers a no-nonsense approach to usability www.useit.com.