As requested, here is my longer review of Secrets of Japan. First off, the author states up front that the book is only the first in a series devoted to Cthulhu Japan, so it might be unfair to judge SoJ as a complete setting (though, for $35 + tax, it pretty damn well should be).
So what do we get for our 35 bucks and change? Well, the book says we should get a "comprehensive portrayal of the culture, history, and people of Japan presented in a Lovecraftian setting." While the culture and people is presented comprehensively, the history is underserved, and the book only works for roleplaying in the modern era (to be fair, it is marked "Cthulhu Now"). I think my biggest gripe is that the Mythos is simply grafted onto Japan, rather than using Japan as a background for a setting with a narrative about the Mythos. This is very much a toolkit of stuff that Keepers can pick and choose from, rather than a rich and coherent setting in and of itself. But that said, it is very much a sourcebook (this is not Goatswood) and should be a useful purchase for any Keeper already planning to set their game in Japan.