I really like this book a lot. While the Class itself is very narrow, it is also really flavorful and fun, appealing to my very sneaky side.
The new feats, skill uses, spells and magic items are really niffty and useful, as well as being unique enough to stand out from all the rest we've seen.
There are however three complaints:
1) No Capstone is a rather large turn off for me. I get that at 20th level you gain Play the Part 7, which is strong, but i see the continuation of an earlier ability to be a very poor last level feature.
Perfect Performance might have been better at 20, but even that lacks some oomph when compared to a lot of the other capstones out there.
2) the lack of any real new equipment. the discussion was neat, but if that's all you were going to include, why not just save that for something else, and add in pages of actual material, or cut those pages entirely?
I understand your desire to include new equipment due to the glut of stuff out there, but this book seems like a great way to introduce really innovative and fun new mundane items that would greatly aid both the Masque and other stealthy characters without needing magic.
3) I wish Bluff had an expanded use. I'll be damned if i can think of anything else, but I doubt i would have thought up of the new uses for the skills that you did include.
overall, even with those three complaints(strong though they may be) this book is definitely going to be used by myself a fair amount, and almost definitely by my other gamer friends for our upcoming game.
4 outta 5 for sure.