The book quickly skims 1920's songs, fashion and general lifestyle and has many hundreds of cool little gems of information -- What was the main newspaper of Amsterdam in the 1920s? What hours was the Library of Congress open to the public? What songs were popular in 1924?
It also has a small but reasonable set of purchasable items like clothes, vehicles and weaponry which the main sourcebook sorely lacks and wonderful information on 1920's forensics and criminal investigation.
Unfortunately, there's a section of occupations in the middle which takes up half the book. There are little nuggets of information like famous members of that occupation in the 1920's -- Woodie Guthrie the Hobo, Howard Carter the Archaeologist and so on -- but overall, it's just a set of additional sets of skills the player can take and extra benefits and powers. None of it is as unbalancing as Unearthed Arcana was to first-edition AD&D, but it's still a step in the wrong direction, and having it take up half the book is pretty silly.
Still, the good far outweighs the bad and unless you're already an expert on the 1920's, you'll find a very large amount here to like and to learn from. If you're only buying one Call of Cthulhu sourcebook beyond the basic, I'd still recommend getting one of the Arkham Country books (Arkham Unveiled, Kingsport: City in the Mists, Return to Dunwich and Escape From Innsmouth) instead of this one since they address specific settings. The Arkham book in particular gives excellent specifics that will be more useful to you than the grand, worldwide events of the 1920's Investigator's Guide.
Of course if you're willing to spend the money, both/all are good purchases.