The Sherlock Holmes novel in which Professor Moriarty is mentioned before “The Final Problem” is The Valley of Fear.
The Valley of Fear was published in book form in 1915, although its events are set earlier in the Holmes chronology. Holmes identifies Professor Moriarty as the hidden criminal mastermind behind a case involving a murder at Birlstone Manor. The reference is significant because it predates Moriarty’s dramatic confrontation with Holmes in “The Final Problem.”
The novel has two distinct sections. The first follows Holmes’s investigation of the Birlstone crime, while the second explains the earlier history of the murdered man and the violent Vermissa Valley organization. This structure makes the book both a detective story and a historical crime narrative.
Readers sometimes assume that “The Final Problem” contains Moriarty’s first mention, but The Valley of Fear was published later while referring to him in the story’s earlier fictional timeline. Publication order and internal chronology therefore differ.