To get this out of the way quickly, I really enjoyed this book. It's cleanly and energetically written, with a great premise and well-realised characters.
Honestly, the blurb covers a great deal of what I was going to discuss as part of explaining why you should read the book.
Mask of the Other covers a number of different characters across several different timeframes, and they are all clearly rendered, interesting, and frequently flawed people. The dialogue was particularly impressive for me, doing a great deal to characterise everyone and how they relate without feeling forced. It's also frequently as funny as it is horrible and profane, and we've all known people like that.
Or at least, I have.
This book is an energetic modern exploration of the Lovecraft mythos, and there are points of easter-egg crossover with A Wind At The Window from the Greg Stolze free fiction library (http://www.gregstolze.com/fiction_library/index.html), and with _iDelta Green/i_.
I enjoyed it, and I'd be interested in a sequel.
As another point of interest, this book has the most imaginative, alien and disturbing rendition of a shoggoth that I've seen anywhere, and that includes Lovecraft's work.
[4 of 5 Stars!]