Cubic: Your friendly-neighborhood-PS forums
are the book! For now, at least.

Many have requested books and online learning for Terragen 2. Your best bet, for now, is to use the search field of the forums with any specific query that you have. You'll likely be surprised at some of the findings you come across this way, aswell.
The documentation section isn't even quite finalized yet here. Most of us just wade-in and see what each setting in each particular node does and then talk about it in here, it'll be confirmed or not within a very little time and usually loads of possibilities open up with just forum talks on settings etc. Tinkering is your future! At least until some final documentation and some books etc. are available.

And even then, still, tinkering is your future! TG is vast.

Enjoy, there's no right or wrong way with TG2, as I'm sure you'll find. Patience is your friend, there're
multitudes of things to play with.
If you haven't read it before, I suggest the following link, it opened up so much about TG2, in general, not just about the power fractal shader, to me.
Volker Harun's Power Fractal Lessons -
http://forums.planetside.co.uk/index.php?topic=2287.0 (
A must read)
Get stuck in!