Nick is of the "old school" of North American bonsai and you will undoubtedly find evidence of that in some of his horticultural commentary. i.e. the minor details.
But you will not find a better written source on info about Larix laricina and Thuja occidentalis. I've personally found that almost every thing he says about collecting, rehabilitating, developing, and maintaining these two species is completely accurate. Those two chapters alone represent about 1/3 of the book.
The horizontalis and communis chapters are priceless as well, although I've not yet worked with those species. Not many people (if any) have studied those species as long as he has. He is also recognized as an authority on Malus sp.
So no, it is not out of date. From my biased perspective as a north eastern collector and bonsai grower, it is the best bonsai book ever written. Not just for the techniques and pest control tips, but because it is very well written and his trees are gorgeous! Plus it is all based on his personal experience, which counts for something. If you live in California, it is a pretty useless book.
I've lost count of how many times I've read the Thuja and Larix chapters, but I find something new every time.