What is the first use in literature of the term 'private eye'?
I would like to put in a bid for Sherlock Holmes. Although he was never called a private eye, he is the doyen of all such. In the Valley of Fear, early in the tale one policeman writes to another "This is for your private eye." meaning 'for your eyes only.' Since he had Sherlock alongside him as he read it and he was asked to share the information with him and to bring him along on the case, it surely here has a double meaning.