Poopsie, you've been lost in the wilderness. About ten years ago D. Robbins published Tarbell 8. The original idea came from Steve Burton, who figured out exactly how much of the material in the original Tarbell System 60-lesson correspondance course never made it into the first 7 volumes. It turns out to have been quite a bit. After he excerpted all of this material and organized it, the publisher brought me in to complete the job, which consisted of gathering additional material of Tarbell's that had never been reprinted, plus whatever else I could scrape up, lay the whole thing out, etc. My goal was to create a book that was almost wholly written and illustrated by Tarbell of material that was not in the first seven volumes.
Thus, Tarbell 8 is a legitimate addition to the series.
Tarbell 9 would have been entirely new material, which I would have had to write and illustrate. My fee for doing the book would not have covered the cost of the illustrations alone, and so I bowed out and returned my advance.
I believe Tarbell 8 will be the final book in the series.