Later, when the humans know how to write a book, it has to think of a way to communicate. New VoiceMail too with Pigeon, or send a message to with the speed horses.
Later, when the man knows how to write a book. I have no idea how to communicate with each other. The new message to the pigeons. Or sending the message to the messenger.
Later, when people know how to write a book. It has been thought of communication. Leave a message with new by pigeons. Or send a message with a message.