Because the subject is the same as another post, I will repeat the same answer...
I am not sure if I would buy the Goi Books series.
What I can tell for sure, that the original Don Books series are highly tuned. I have one of the latest versions of that book and it is very high tuned.
The author has his blog here: https://donvasid.blogspot.com/2019/11/don-book.html.
I ordered it and he was very polite and quick.
Other sources, which are "selling a Don book", is just a crap.
I am less convinced and less convinced that I should stay in this Forum....