Who is de Selby from The Third Policeman and what is their importance?

De Selby is a philosopher. The narrator first reads de Selby's works while he is in school, and all of the narrator's sins stem from his obsession with de Selby. The narrator first steals de Selby's book. Then, he allows Divney to take advantage of him and his farm to write his own book about de Selby. Finally, to get his book published, the narrator resorts to murder.