It really depends what you mean by usability. A lot of people say this when they actually mean very much more wide-ranging user studies. I take usability studies to mean simply testing out facets of navigation and web design towards the end of a project. This is good, but it's better to do a proper study, starting as early as possible. So I wouldn't use that as a key word: try users, user study, etc.
My colleagues and I have done a lot of work in this area, so take a look at my publications page at http://www.ucl.ac.uk/dis/people/clairewarwick/publications/
You should also look at Stan Ruecker's publications: he's done some fascinating work with users, especially on the Orlando project and the MONK and NORA projects. It's worth looking at MONK's webpage as well for details of their work on use cases. This is different from user studies, but can also be helpful. Stan's just written a great book on interface design, which should be very useful.
The LLC volume of conference proceedings for DH2009 includes two interesting articles by Dana Wheeles and John Keating about their work on users.
More widely you will probably need to consult literature written from an HCI perspective. If you are looking for a good overview of this then try Schneiderman and Plaisant's very helpful book on this "Designing the User Interface". Of course for web usability you should also look at Nielson's useit.com where you'll find a lot of useful resources including the classic article on testing with five users.
I'm also writing an overview chapter on users for our forthcoming book DH in Practice. It's not published until next year, but I'll be able to send you a draft chapter soon if you email me.
Best wishes,
Claire
