Breaking the Spell is supposed to be an examination of religion, including its causes and results, maybe looking at whether overall religion is a negative or positive impact. Dennett is a notable atheist, but in Breaking the Spell he sort of tries not to write in a pro-atheism manner. It's not very convincing. He pretended to be unbiased, but it was abundantly clear throughout that the book was not a disinterested analysis of religion, but largely for its demise.
The audience Dennett thought he was writing for is incredibly unclear. It certainly wasn't for well-read atheists or anyone who has read more relevant things about the origins and effects associated with religion. Dennett claims that the book is written towards religious people, hoping they will read it and step back and really look at their religion. Somehow I don't see this happening. You wouldn't even have to be very dogmatic to automatically reject Dennett's ideas in this book, his writing makes it almost necessary to reject what he says, somehow! A typical, "unbiased" passage from Breaking the Spell:
It might be that the best that can be said for religion is that it helps some people achieve the level of citizenship and morality typically found in brights (atheists)." (page 55)His little hypothetical judgments were not very helpful, and certainly reflected his views, regardless of how disinterested he was trying to come off. So many things in this book are well suited to driving away anyone not already in his camp.
This book was also very rambling, repetitive and boring. It never really got anywhere, it mostly just kept saying, "this is what I'll show" without showing much. This was not a conclusive, persuasive treatise on why atheism is better than religion, although maybe that's what Dennett wanted it to be. There were many, many times I finished a paragraph (or page, or chapter...) and realized I didn't remember the point of what I had just read. It was written in such a way that the point of many parts of the book was really unclear.
In summary, this book is a big meh. I wouldn't recommend it to anyone, religious, atheist, or whatever. It's just an epic waste of time that would probably really irritate a religious person for no good reason!