Reviewing a roleplaying game (RPG) book is quite different than reviewing a fiction or non-fiction book; they are still storybooks in a way, but also instruction manuals, rule books, reference tomes and guidebooks to the setting - the world in which the game takes place. It would be unusual to read it cover to cover, you would tend to read sections some several times and dip into others, returning later for reference. The criteria by which they are judged is, therefore, quite unique.
World War Cthulhu: The Darkest Hour is a primary setting guide for a pre-existing game. Call of Cthulhu is a bona fide classic of the genre, one of the first games to move roleplaying away from the hack n slash of Dungeons and Dragons and its ilk into a more investigative, thinky, character-driven milieu. It is based of the Cthulhu Mythos writings (primarily by H.P. Lovecraft, but expanded by others) and I don't think it's any coincidence that it is since that games' 1981 launch that Lovecraft's work has been rediscovered by an ever wider audience.
The primary setting for the original Call of Cthulhu is, as with Lovecraft's stories, New England in the 1920s (with adventures in exotic places such as Antarctica and the Amazon), though the horrors can be transplanted to any era or locale, and there are many supplemental books that do so. World War Cthulhu, as you can probably guess, is a guide for adventures in World War II, and is excellently done. As well as their standard military roles (for the Allied forces, naturally) the characters are agents for a shadowy civil servant who tasks them with investigating potential extra-dimensional spookiness - at the same time as having to survive the very real horror of the bloodiest conflict in the history of humanity.
As such, the game can be run leaning either way; classic Cthulhu-style, using the war as a backdrop, or primarily focusing on the war itself with occasional added horror (imagine some other terror invading the trenches, like the zombies in the movie Deathwatch) or some mixture of the two.
As a supplement to Call of Cthulhu, this is excellent. There is lots of great background on the various intelligence agencies (there is, understandably, a push to make the characters intelligence operatives rather than straightforward soldiers, as investigation - often behind enemy lines is such a large part of the game) and great short summaries of the theatres across war-torn Europe and North Africa.; general information on the status of various countries as well at least three or four adventure ideas for each as well as a list of "Fortean events" through out the period for extra inspiration. There are, of course, equipment guides (description and statistics for a wide range of WWII weapons, prices for both legal and black market goods, etc), background on some historical figures and how they might be connected (Ian Fleming, Dennis Wheatley, Kim Philby, Aleister Crowley, and others) and, of course, an update of the Dark, Lamentable Catalogue - a guide the Elder Gods and other entities, as well as various Earthly factions and cults. As is now usual, the book ends with a fully fledged adventure to get you going - although extra supplements, including adventures, have already begun to appear. i have not yet run this adventure, entitled The God in the Woods, as I have a couple of my devising to get the group into the swing of things, but it's always good to have one as back up and it looks to be a good one.
The book is very well put together - a solid, attractive hardback (build quality is a consideration for RPG books, as they will get a lot of handling while using for reference) with a good contents page and index, something that many RPGs still lack, an unforgivable omission for any sort of reference book - with good artwork and layout, double columned throughout with good placement of tables and highlighted boxes for important text and examples (again, the number of game books where a table turns up many pages away from the section to which it refers is quite astounding. There is an old joke that RPG designers have an excellent vocabulary that unfortunately doesn't include the word 'proofreader'). The writing itself is solid and, importantly, clear.
This book is produced by a company called Cubicle 7, from whom I have bought quite a few books in the past (they produce the utterly superb Laundry Files RPG, based on Charles Stross' wonderful books) and they have quickly gained a reputation for quality and attention to detail. There's always a good amount of free downloadable content on the website (character sheets, tables and even adventures) and I can personally vouch for their superb customer service; when you buy the physical book you get a PDF as well and they one time I had to chase up the link I received an immediate response.
A great advantage to this setting, of course, is the vast amount of material available online. I have researched locales and buildings (I needed a ruined castle in Poland for my adventure) along with timelines of the war to plan my campaign and the location of adventures.
Highly recommended for your insanity inducing, squamous horror laden, world changing conflict pleasure.