I've been curious about this book since the first Conjuring movie. Satan's Harvest covers the possession of Maurice Theriault, a French-Canadian farmer. His possession is considered one of the most publicized and one of the few exorcisms actually recorded.
Now, just to get this out of the way, I do believe Theriault was possessed due to the footage of the exorcism that I have seen. It's an interesting video and due to the time period I'm skeptical it was faked. Not impossible, just not convinced. That said, if I had to use this book to make that decision, I'd say both Theriault and the Warrens are major frauds.
Despite claims of photographic and video evidence, there were very few photos taken in the book. Most were just of Ed and Lorraine, actually, which I felt contrasted very differently from the other books of theirs that I had read. I hate when paranormal books do that, saying they have evidence but never showing it. I'm reading this book to learn and be convinced. The author is the one making the claims so they better do all they can to convince me.
The Warrens themselves, who I've always considered a decent enough couple, came across very differently in this book, especially Ed. He seemed arrogant, pretentious, and close minded. I was especially chuffed during the chapter on the psychiatrist's interview with Maurice. This is a man who went through incredible abuse throughout his life and I was genuinely interested in what those findings would be. But Ed basically shot down everything the man said if it contradicted the theory of possession. This portrayal of the Warrens is very in line with criticisms skeptics pose about them. If this book were my first exposure to them, I'd be fully convinced they were frauds as well.
The book also glossed over some pretty major factors of Maurice's life, namely his criminal charges. During the times where his possession was most blatant, Maurice was accused of arson and sexually assaulting a child. You would think the book might want to explore that a bit more, given that it's awfully convenient that he's possessed and therefore wasn't in control of his actions (/sarcasm). Additionally, his treatment of his first wife sounded rather awful and I didn't appreciate how sympathetic the book portrayed him. You can be possessed and still also just be an asshole.
Like I said, I do believe something paranormal was happening with Maurice. I also think he is fully responsible for at least the sexual assault and probably the fires. The Warrens say so themselves, usually someone has to either invite the spirit in or be doing something incredibly amoral to open themselves to demonic possession. At the very least it's a Chicken or the Egg kind of question about his possession and I really wish the book had explored it more.
Final rating: 4 out of 5 stars. A lot of fun spooky stuff to read but oddly narrow minded compared to the other books I've read by the Warrens.