When running Cthuhlu, I have a firm desire to stick predominantly with the Victorian age. Foggy London back alleys, dark cults and secret societies hiding under a veneer of respectability all call strongly to my sensibilities.
The '1920's Investigator's Companion', though, has gone a long way to making me think about alternate timelines to run the game in. Written in the usual informative style and peppered with 'authentic' images, the book does a great job of evoking a sense of another time. The sidebars are just as useful and stand out to give the Keeper a bit of extra flavour when designing their game, or running a set module from this time period.
The book is divided into four sections:
Part One provides a historical overview of the 'Roaring '20's' and notes about most of the major occurrences of the decade. The game information really comes to the fore in Part Two, which describes specific occupations, extra rules and some pointers for playing a character in the 1920's. Part Three, which provides and overview of the technology, science and culture of the time is my favourite part of any Cthulhu book - and this one doesn't disappoint. The section on research tools was especially illuminating and there are plenty of prime examples which an enterprising Keeper can use to bring disparate information resources to the attention of the investigators. Whilst the further notes on transportation, general equipment and weapons were interesting, it was the former section with held my attention. It would be a good counterpoint to modern games where information is usually seen as a ubiquitous, highly accessible resource - in Cthuhlu, the secrets should be a little harder to find and put together (although we know the price of doing so).
Part Four is a must-read for any player, presenting a range of tips and advice for playing an investigator. There are hints on effective questioning techniques, licensing, using force and firearms, and an overview of the state of forensic science in 1920 (which is a great read by itself).
In all, this is a brilliant book and up to the standards I expect from Chaosium. Cthuhlu has been running for a couple of decades now, and with sourcebooks like this is isn't hard to see why. I'd recommend this for the shelf of all Cthuhlu Keepers.