In TOS: The City on the Edge of Forever, Kirk, having traveled back to the 1930s, says:
Let me help. A hundred years or so from now, I believe, a famous novelist will write a classic using that theme. He'll recommend those three words even over I love you.
A literal interpretation could place this book into the 2030s, but it could also refer to an earlier time. The episode aired in 1967.
Was Kirk referencing some actual book that exists, or did the writers invent a hypothetical influential book that was yet to be written?