A List Apart's Application Cache is a Douchebag is great. It's well-written, filled with lots of useful details, and easy to read. Of course, it's also titled in a way that some find offensive (although I do think that in North America, the word "Douchebag" is used very often and has lost much of its sting).

I've been doing some work on an offline web application with one of my clients, and this article has already saved us some time, so if you're considering going down this path, I recommend you give the article a read.

I'd also note that if you're going to do any work with Local Storage, you'll probably want to do some profiling, probably with Firebug, inside Firefox, whose local storage implementation has some rough edges -- in particular, these methods:

localStorage.length
localStorage.getKey()

Both of these methods have a tendency to slow down pretty quickly as the number of keys mount in Firefox.