Good news; re Geometric Dimensioning and Tolerancing There is truly only one interpretation of that subject, and it seems to have been conveyed endless times. If you were to work for many large companies, they'll have reproduced it on their own. It's possible whoever created it did so pro-bono. Anyone selling at more than a few bucks is just un-informed of quantity available and distribution. Because subject matter is beyond them, so eyeballs turn into $$ signs.
Since it is not theory, but practice, same material is found within many texts on design, drafting, engineering, manufacturing, process control, etc.