It is proving difficult to keep up with all the changes in Google Apps. I suppose in some ways this is a good thing because it shows that the whole area is moving forward. However, it complicates matters when you're trying to write about it. One change of immediate significance is that GUI Builder is being phased out. Quote "The GUI Builder will continue to be available until September 9, 2013. After that point, you will not be able to create or manage GUI Builder components, although existing components will still function."
The Html Service is the wave of the future so I'm planning to read up on this and I'll do a posting on it soon. One real positive I've taken from the Google link above is that Html Service can be used with JQuery and JQuery UI. That's a real boon to Google Apps developers in that the skills gained will be transferable to wider web development.
Another link with some more information.