TL;DR: A stylish book made with the promise of jump starting a Mage game of Lovecraftian horror-mystery, but only makes use of some Mythos-inspired set dressing.
Heirs to the Mountains of Madness is a name filled with the promise of Lovecraftian horror- but instead, mostly relies on Lovecraftian references. In Providence, RI, a Technocratic Amalgam and Tradition Chantry have both gone dark to their respective larger organizations, fallen under the sway of an ill-omened book. Said book claims to depict the means of creating a Nephrandic Caul- as well as a means to reverse the process.
As a hook for a Mage game, that's totally workable! But it's using the surface most trappings of Lovecraft's works, rather than either attempting to set up cosmic nihilistic horror, or introducing creatures or phenomena beyond the ken of man or Mage. I was hoping to see things like the Zigg'raugglurr (fourth dimensional entities that do crazy time-and-space shenanigans), or locations where the laws of reality (consensual or otherwise) were implied to be breaking down, and how that might change the way Awakened Magic works.
(An exception to this is the Ecstatic Mage, Walter Gilman, who has been possessed / is being transformed / consumed by some kind of intelligent spore, which is brilliant and the sort of creepy thing I'd want more of in a book with this name and concept)
Additionally, the NPC stats of the Mages are kind of wonky and on the rather high side. Arete ranges mostly from 4 to 6, with every character at least one dot in almost every or actually every Sphere, as well as either Mastery in one or multiple Adept rankings. (Poor Walter Gilman, again a Cult of Ecstasy member, and the only Mage in the book with no dots of Time- although this looks like it might have been an editing error)
The book looks like a professional White Wolf product; good use of the template, generally well-chosen art. The only quibble I have in terms of style is with a real photo used for one of the character portraits, while line art is used for all the others; the difference jumps out and is kind of glaring.
[3 of 5 Stars!]