The first has to be ARCHIdigm - Odin's free online tutorials showing the extra tips you won't find in the Help section. If you want a serious handle on ADT/ACA and it's capabilites then you have to work through every example on his site. He provides lots of real world examples and shows a seriously devious mind in working around every limitation that you come up against. I bet he pulled all his toys apart when he was a kid. I have a BIG debt to his articles to getting under the hood. I desire to emulate his easy to follow style and put it into the aussie context of how we do things here. Seriously, go thru EVERY article. You may also be able to purchase and use his content without too much fuss in a metric environment as one (Spanish) user already reports.
http://www.caddigest.com/subjects/adt/tutorials.htm is a great collection of articles from all over, work thru them when you have a spare moment :-) or search thru for specific topics.
I couldn't post it cause I couldn't find it but Ha! it's one of my own links below!! Learn ADT is an excellant resource. It's amazing how clear things become when you watch a video presentation.
Another site that have SIGNIFICANT content available to view or download are http://www.visionrez.com/ and pick on the support and education and then video library. Now lots here relates to their own add on product and you should be aware of it's potential, but also there is lots of info that just relates to ACA/ADT. (Hope that's all right Jay). If we can get these guys to Australia with their own standalone product (based on ACA) then I believe it will make a big splash.
So whilst I am working on tidying up my books for the ATO, if you haven't found them already, get into these links and raise your ACA experience to a new level.