Blaze72: Before i was doing it inside of 7-zip
Just tried the above way and i get a error message "Can not open file 'C:\GOG Games\Powerslave\game.gog as a archive
there's also issues with BuildGDX too and i am running the latest version, Lots of Java related error's.
Just to be sure, you can launch the included GOG "Launch Powerslave" shortcut and play the game correct? If not you may need to verify/repair.
As far as BuildGDX goes, I understand there's 2 versions. A Windows 10 64bit build that uses an included Java and the other that needs Java externally. Have you tried updating java if you're using the external version?
*Just remembered that I installed the following plugin for 7-zip a long time ago. That's likely the reason I can access it and you can't.
https://www.tc4shell.com/en/7zip/iso7z/
If that doesn't work or you don't want to use it, then here's another way of getting book.mov.
In the Powerslave folder make a copy of "dosbox_powerslave_single.conf". (You'll need the copy to restore after)
In the original replace the contents (using notepad) with the following below:
[autoexec]
@echo off
cls
mount C ".."
imgmount d "..\game.ins" -t iso
cls
d:
copy book.mov c:
echo Done! book.mov has been copied
pause
exit
Run the "Launch Powerslave" shortcut, then delete this "dosbox_powerslave_single.conf" and restore your original copy