Written completely in-character, this work is subtitled "The Investigator's Guide to Occult London" and is purportedly based on the research of the recently deceased Augustus Darcy. Perhaps, if you are using Bookhounds of London, this has turned up in your characters' bookshop. Or, in any campaign, it has come to the Investigators' attention in some manner, a useful resource if their investigations take them to London.
If you are the Keeper, a thorough study of this tome will reap great rewards in terms of local colour and the myriad ideas for plots that will spawn as you read through it. Do, however, share it with your players; let their characters consult it during the game or even let them read it at their leisure between games.
The main part is a discussion of places, an occult gazetteer. Divided into geographical areas (beginning with the Square Mile of the City of London itself), entries are then alphabetical, making it relatively easy to find the one that you want. For each location there are notes on relevant occult connections (note: occult, rather than Mythos), with frequent references to ghost sightings, inexplicable feelings or smells and the like. Much lies unexplained - and that's the fun part, any one might become the basis of one of your plots, or at least be part and parcel of it. You barely need to lay out the clues, they are here for the party to find for themselves! Period images mingle with snippets of lore at every turn.
There is also a section on various people, some historical like John Dee, others supposedly contemporary figures of the occult scene. Perhaps the Investigators will encounter them in their travels, or seek one out if their knowledge is appropriate to the matter in hand. Again, reading many of these entries suggest encounters and plots a-plenty.
As a casual read this is a fascinating work. Even if you are not a role-player, or prefer other genres of game, it makes for an entertaining read if you have an interest in occult lore or indeed if you know your way around London - if you are not near to there, you can pull up most of the locations on Google Maps! (Or if you have Bookhounds of London, the extensive 1930s London maps therein will come in handy in orienting yourself.) If you do play any Myths-related game, or one based in the 1930s or thereabouts, it becomes an invaluable in-character resource.