The book is currently in writing.
The reason DotNetNuke appears to be slightly more challenging that Joomla is that is so much more flexible in what you can do.
Have you looked at the skinning documentation to get a start on it. Have you downloaded it, printed it out and looked through it? I have a version of it online in a CHM Help format which registered users can download from my site for free.
I got my start from reading that very documentation. No one helped me and I'm not a developer, but I can understand code.
My suggestion is that you download some free skins and have a look at how they are done as well.
You can also open the dnn-blue folder, look at the default skins in there, and see the html files, the ascx files, the skin packages, the css file, the xml file - and hand in hand with the documentation should give you enough information to get started.
However, you will need to stop thinking about other CMS products, since that's where alot of comparisons come in to DNN, what sets DNN apart from all the others, regardless of the fact it's got some headaches from time to time like any software product, it's very powerful in changing the look and feel of.
I'm glad you've mentioned *stiff guidelines* because I've advocated that for some time. Yes they are stiff guidelines, but wow, once you get the *penny dropped* factor, it's scope to work your designs into a completely dynamic website that no one can pick is DNN is simply amazing.
I'm going to be sending out some newsletters soon - on weekend, announcing some very cool information about learning skinning from 3 professionals who have teamed together to produce a new direction for learning.
I expect we'll see something on Amazon in September for pre purchase. The date of the book release will be set by the publishers soon.
Please read the documents. They are a good start for you.
Nina Meiers