The Secret at the Hermitage by Carolyn Keene

Delusional Harold Norton, ex-warden of the Pinecrest Reformatory, accuses Louise Dana of being escapee Nina Regan. Louise clears herself, only to find that Norton continues to stalk her every move. After Louise is attacked by Norton, she is lost in the woods and spends the night with a strange hermit.

Upon Louise's return to Starhurst, she becomes intrigued with Nina's strange case. Nina was never proved guilty of her crime, so Louise wants to help her. The girls must discover Nina's whereabouts -- and prove her guilt of innocence!