What is the nature of a message that is destined for two or more addressees, but no addressee needs to know about the others?

The nature of a message destined for multiple addressees, where no individual addressee needs to be aware of the others, is accurately described as a private message. This type of communication allows for confidentiality, as it ensures that each recipient receives their own version of the message without knowledge of who else is receiving it.

Private messages are essential in scenarios where personal or sensitive information is communicated, maintaining the privacy and discretion of each individual involved. Unlike a group message—which implies awareness of other recipients or a communal setting—and a broadcast message that may be intended for a broad audience, the private message is designed specifically for individualized correspondence. Thus, each recipient’s focus is solely on the message intended for them, without the context of the other addressees involved.

Choosing "book" would not align with the principles of privacy in communication, as a book is typically a public source of information and not directed at specific individuals.