Practical Instruction for Detectives oleh Emmerson W. Manning

First published in 1916. According to the Preface: " Having been connected for many years with two of the largest and most successful private detective agencies in this country, both as an operator and as an official, and having been requested to outline briefly and concisely the most modern and up-to-date methods employed by leading detectives and private detective agencies of today, I shall confine myself in these pages to facts and a few personal experiences. I will endeavor to show that any person possessed of average intelligence, and who will use good common sense, can become a successful detective, regardless of his present or previous occupation."