The Shadow over Innsmouth is probably Lovecraft's most famous story after The Call of Cthulhu and certainly the most influential. Most readers of horror know the basic outline and almost all Lovecraft fans. A New England town has taken to worshiping Dagon and breeding with Deep Ones in exchange for gold as well as fish. This is due to a centuries-old compact between the Deep Ones and humans negotiated by Obed Marsh. How a man could become involved in such a monstrous Faustian bargain was left vague and Matthew Davenport has decided to fill in the blanks with Obed's autobiography.
I was really looking forward to this as Obed Marsh is one of the most infamous figures in the Cthulhu Mythos, being one of the few named humans of any relevance. Aside from Joseph Curwen, I can't think of any who really warranted attention as a villainous character. However, it's something which could have easily gone wrong as it would be easy to make his actions seem reasonable or make him a caricatured villain. Such a vile compact as selling your town to the cult of Cthulhu needed a man who was both monstrous and yet not a cartoon.
The author, here, crafts a wonderful tale about his hypothetical tale of corruption and decadence. I'm a huge fan of indie fiction, especially indie horror which Lovecraft himself would have qualified for in his lifetime. I strongly recommend this particular tale which makes me sympathetic for a man who damned his entire hometown. The one in the book was a heroic seeming figure who just compromised far too much--and gave away his soul one bit at a time.
Interestingly, I have to give Matthew Davenport credit for the fact while Obed Marsh has understanding motivations, the story doesn't attempt to redeem the character. Every step along the road to hell (by way of R'lyeh) is done with an endless series of rationalizations and self-justifications. Obed Marsh tolerates the human sacrifice of the Cthulhu-worshiping Islanders because "the town depends on him", he participates in their demonic sacrifices because it's the only way to keep the gold flowing, and he's not betraying God because of keeping God in his heart.
In short, Obed Marsh is a horrific hypocrite but the kind which is all too believable. The world is full of people who have the belief they are good people but who profit off the suffering of others. Obed Marsh is never a believer in Cthulhu (identified as Dagon here as some interpretations have done). Instead, he is an individual who can't see past the gold and prosperity of the islanders even as it's increasingly clear they're crazy cultists. Obed Marsh equates success and position to goodness and like so many other rich men in America (or other nations), uses it as a way to show he's doing the right thing.
Not a common thing in Lovecraft stories.
This book really doesn't dwell upon the luridness of humans breeding with Deep Ones. There's a scene where a woman assaulted by a Deep One is forced to carry the child to term by priests but it's dealt with in an off-handed manner. Obed doesn't care what happens to his people as long as it's out of sight and out of mind. I think this works well as it reflects Obed's own ability to paper over the horribleness of his actions. The finale where his descendants take their brides among the Deep Ones also reflects how they have become true believers--something we knew would happen because of The Shadow over Innsmouth.
If the book has any flaws, it is a bit too predictable in its movements. There's no moments where Obed makes a fleeting attempt at redemption or twist like he was possessed by Deep Ones. These kind of things pop up in stories about predestined events and are oftentimes cheats. Still, I think the author could have taken a few more risks with Obed Marsh's story.
Ultimately, this book is more about the journey than the destination and there's very few twists and turns throughout. I enjoyed the final end of Obed Marsh's long and storied career as Cthulhu's high priest, though, because it does show that he had some small part which belonged to him. It was also a bit of cosmic irony which I'm sure Cthulhu (or at least Nyarlathotep) would appreciate.