Since it is a new program in its current incarnation, pretty much all that has been written about it is the manual, and this forum.
However, there are people on the forum almost full time answering questions and listening to input.
Like auto-cad, Double-cad is a bit of a steep learning curve at first.
That said, the basics are similar, so if you can find a basic autocad book, much of what is in there in terms of generally how to do stuff would apply. For some of the command specifics, the manual for this program would be the first place to look.
Also, much of what you do learn would definitely be useful, if someday someone stuck you in front of an autocad setup.