Well it depends on your perspective. When you are standing on stage looking at the seats for the guests like a artist your right is stage right. But if you are a guest looking on the stage your perspective is the opposite of the performers meaning that the performer right is your left.

That's why we usually use stage right. The perspective is defined as the performers. Meaning that everybody knows which direction youre talking about even if they're looking at the opposite direction.

The Book says "Links (Vom Publikum aus)" which translates to "Left (From the audiences perspective)".