If you can use a Java backend or if you don't mind deploying to Google App Engine, you might want to take a look at Google Web Toolkit. They have excellent tutorials and example applications, including a Mail app that you might be able to modify to suit your needs.
If you'd rather not use GWT, you can use any of the several JavaScript UI Libraries out there along with any backend; YUI, Ext, jQuery UI, MochaUI, to name a few. You could also roll your own UI using the underlying cross-browser libraries (YUI, Ext JS, jQuery, MooTools, etc.) or even just plain old JavaScript/HTML/CSS/DOM.
It all depends on your requirements, how much control you want over your code, look-and-feel, behaviour and what your preference is.
Ideally, you'll want to brush up on JavaScript fundamentals and Object Oriented JavaScript as well as HTML/CSS and your server-side language, so that you can build an application with structured, clean and maintainable code. Books on the topic are easy to find.
Realistically, just find a tutorial in the development stack of your choice and continue from there.
It would be best to try and strike a balance of the two.