well its got to be some sort of higher level spell because it allows communication between two separate objects no matter the distance or dimensional world. I don't think there are more though, if there were I mean what reason would you have to go and actually speak with another pony. I do wonder though is the connection between only two books or could you add more books and essentially have some sort of group chat?
Kinda a different question but also about Starlight, did anypony notice that Starlight's magical aura was a different color in this episode than it was in the first movie?