Warren is referred to in other Lovecraft stories. In The Silver Key, he is alluded to as a man in the south, who was shunned and feared for the blasphemous things he read in prehistoric books and clay tablets smuggled from India and Arabia. In Through the Gates of the Silver Key, he is revealed to have come from South Carolina, and to have been an expert linguist of the primal Naacal language of the Himalayas. Warren is also mentioned in Brian Lumley's Titus Crow series, as a member of a Bostonian group of psychics.