Principles of Bonsai Design by David DeGroot is an excellent book that covers a wide range of bonsai styling as well as horticultural elements. Would recommend for any bonsai enthusiast. It gives examples of in ground trees that could possibly be considered niwaki though I am unfamiliar with the term. You could no doubt apply the tree styles and rules of visual balance on a larger scale in relation to your in-ground trees. I was able to acquire a hardcover copy on the internet for $32.

I will have to go visit Nicks at some point, although Its just about across town from me

I recently saw this video too and I'm going to give it a try with some silver maple cuttings from a silver maple in my backyard once the cuttings are ready. Anyone have experience with silver maples? I know they are very good at regenerating branches since we've cut the tree in our backyard to nothing but the trunk 3 times in the past 7 years and its come back strong every time. I see no reason why they would be very different from other maple species but sometimes trees can surprise you. Edit: it has come to my attention that the leaf size will be quite large and will take significant effort to shrink if its even possible to get it to an appealing size at all. This being said I'm not worried about it since it will still look nice in the winter or when defoliated.