Web Application UI Guidelines?

Tom Davies asks: "Every GUI platform has a document which describes the conventions developers should use when building GUIs with that toolset. There are also lots of good resources for Web site usability (e.g. Jakob Nielsen's useit.com). But what about web -applications-? I am developing an intranet application which is aimed at people who use it every day, not those who stumble on a web site and need to be able to use it immediately. It can have a higher learning curve, but must deliver more 'client/server like' usability. Are there any sites/books which address how to make HTML GUIs for applications?"