Lucy seeing the book in the 2005 movie.

"Is Man A Myth?" was the title of one of the books that the Faun Mr.Tumnus had on his bookshelf in his cave.

Lucy Pevensie noticed the book when she first visited him shortly after first arriving in the world of Narnia.

Presumably, this book was written by a Narnian at some point during the Age of Winter, when the White Witch ruled Narnia. After so many years of never seeing a human, some Narnians would have undoubtedly become skeptical over whether humans actually existed or not.