Mr. Tumnus' books provide the reader with clues about Narnia, its history, and its population.
The Life and Letters of Silenus - Silenus is a character from Greek mythology, a companion of Dionysus. He is often depicted as being a father or forefather of nymphs and satyrs. That Mr. Tumnus has his biography indicates that in Narnia Silenus was real. For a depiction of Silenus from our world, see Euripides' Cyclops.
Nymphs and Their Ways - Nymphs are nature spirits, usually presiding over springs and rivers. This study tells us that in Narnia nymphs are real.
Men, Monks, and Gamekeepers; a Study in Popular Legend - The title of this volume tells us that humans are not denizens of Narnia; they may have once existed, but there is little to indicate this, and the author of this volume obviously considers humans and their occupations to be imaginary.
Is Man a Myth? - Again, the title of the book indicates that there are no men, or humans, in Narnia per se, but that there have been traces or legends of their presence in Narnia's past.