Anyone new to Call of Cthulhu, especially those coming from the D&D world, could be forgiven for thinking the game might benefit from a "Monster Manual".
I can tell such people that because of the completeness of the rulebook back to the earliest versions, the need for this volume is imaginary. The latest version of The Call of Cthulhu has a bestiary which should fuel any campaign for years (remember that Mythos Creatures are often repeat performers, not single-episode walk on targets for players to coat with arrows and fireballs). I have a very extensive collection of published scenarios and campaigns for the game and I can't recall ever seeing anything in them that wasn't either in the rulebook or described in the publication itself.
That said, this volume contains a staggering amount of information on just about every monstrous thing that should not be that has ever waved a tentacle or pulsed with colors unknown to earthly beings. A Keeper/GM will find a wealth of useful information with which to pad out his bad-guy arsenal.
The incidental artwork is superb too, depicting circus posters, tarot cards, Mesoamerican antiquities and so forth, all subtly (and not-so subtly) altered in some vile mythos fashion. I found myself doing spit-takes on more than one occasion while browsing through it.
Production-wise, what you have is a perfect-bound paperback, with black and white printing throughout on almost 300 pages. The only color you'll find is on the cover. Personally, I wish they'd put out this one as a hardback with stitched signatures and a cloth binding so that it would be easier to use in-game and harder wearing, but if wishes were pennies we'd all be living on our own Tropical Islands. Should Chaosium ever make a hardcover version, I would buy it (and I already have this volume *and* several versions of the rulebook).
As it stands it is good value for money as these things go, notwithstanding my earlier assertion that this is definitely a Call of Cthulhu luxury item rather than an essential addition to the core rulebook (there is in fact no such thing - the rulebook, some RPG supplies and a few friends are all you'll need).
I would even go out on a limb and say that the Trail of Cthulhu GM and the Realms of Cthulhu GM would also find this volume of interest, if not so immediately usable without some conversion work.