DOYLE, Arthur Conan (1859-1930). Autograph letter signed ('A. Conan Doyle') to an unidentified recipient, 15 Buckingham Palace Mansions, 10 June n.y., ABOUT THE OSCAR SLATER CASE, relating his attempts to locate the crucial witness Helen Lambie via the Burns detective agency, 'They are famous detectives and I started them on a hot trail but it soon ran to nothing. She is of course the vital witness, and our efforts to find her show how little we fear her', also endorsing a statement purported to be by her, one page, 4to; [with] an album leaf signed by Oscar Slater with the sentiment 'A friend in need Is a friend indeed', 26 July 1928. (2)