In Myst, players quickly discover that the brothers Sirrus and Achenar are trapped in books, and they can see and communicate with the player through the book:
I vaguely recall these being referred to in the journals you find as "prison ages" intended to trap greedy explorers. It is possible to get an ending where you are trapped in the age, where it's all dark except for the window you peer out of at any outside observers.
This matches how the prison ages are portrayed in Riven and plays a pivotal role in that
you must enter a prison age in order to trick Gehn into following you into one, and it is dark except the window, just like the other one.
In Myst IV: Revelation, we visit these prison ages. Sirrus (red book) was trapped in Spire and Achenar (blue book) was trapped in Haven. However, they are entirely different in that instead of being a pitch black area, they are entire ages with large landscapes, and there isn't any way to communicate with the outside world. Certainly nothing that seems to indicate that there is a window to talk through.
So why do the prison ages differ in Myst IV: Revelation compared to Myst and Riven?