I have asked Yanosey directly on this. When the book came out, sales were "so-so", but since then there seems to be more interest in this road. We'll see what he has to say.
One of the problems is getting more good material. When I did the guide, I had about a five hundred or so photos to work from. (In fact, this was a pleasant surprise, as I assumed not too many folks shot PC cars.) The best went into that book, leaving also-rans and other stuff not really first rate. Where will more images come from? Yanosey now wants as many photos in each book as possible (used to be limited to about 300) because cost of reproducing has dropped. So, if my color guide work has been any indication, we'll need yet another 400-500 to work from, assuming the normal percentages apply (good film vs. second rate). But, on the good neews side of the coin, we can now use dupes with pretty good success (heck, I know which ones are dupes in my books and often I cannot tell when the finished product arrives!) so perhaps more will allow their work to be used if they don't have to worry about originals.