If you've got a bit of common sense about software and can already kind of talk about some of the issues like piracy, viruses, etc - then the Excel book is all you need.
it's the only book i used and i did alright in the hsc (2006) - it's written really clearly without the waffle that the others use
when you go through it, you realise how repetitive the software course is, and that you can pretty much study for the exam the day before.
the most important thing with sdd is learning all the terminology that they use and then being able to throw it into your exam responses (even if the question didn't specifically ask for it)