Total Gym Training

I've been using the Total Gym for a few months now after being introduced to it by my physical therapist. It works well for someone with a very bad back. My key question is "why, with a fitness device thats been out over 20 years, are there no books on this thing?" For heaven's sake, there is just one video out on it not made by the Total
Gym folks. I know there are "hundreds of different exercises" that you can do with this thing (my therapist has the "big book" for physical therapy) but there is squat for the home user. Any ideas or pointers?

You observe astutely.
And the reason is really very simple.
It doesn't really matter whether a product is legitimate, bogus, works, dudn't work--dudn't matter. All the company is looking for is revenue. The point being: TG has no idea what is legit or non-legit about their product, and they don't really care, as long as revenue goals are met.
If it happens that something *is* (miraculously) legitimate, well then great.
But it makes absolutely no difference to the marketing company, except for whatever value good "buzz" has. No buzz, or bad buzz just ups the marketing costs.
The TG is notable for two exceptionally useful exercises, the back-friendly military press, and a versatile knee-bend squat. The "100's" of other exercises are mostly redundant, and not especially useful in light of what else is out there.
I am curious as to your PTs "big book". Does it list general exercises, which he feels can be done on a TG, or does it mention the TG/TG genre specifically?