The 1920s Investigator's Companion (1997)

From the back cover:

An Essential Guide for Players

When faced with the horrors of the Cthulhu Mythos, investigators need all the help that they can get. This essential player's aid for Call of Cthulhu provides it.

The 1920s Investigator's Companion is split into four sections. "The Roaring Twenties" details life in the 1920s, from a general historical overview to listing of favorite songs, books, and films of the era. "On Becoming An Investigator" details the trials of becoming an investigator, offers 140 different occupations, and annotates the use of skills in the 1920s. "The Tools of the Trade" lists resources investigators may use for research, describes various forms of transport and transportation, and also catalog other equipment and weapons. "Words of Wisdom" brings the book to a conclusion by offering advice to the intrepid investigator. Now, for the first time, everything a 1920s investigator needs is gathered in one place.

Was published as two books; Investigators' Companion, Volume 1: Equipment & Resources and Investigators' Companion, Volume 2: Occupations & Skills.