This book may very well be the worst book I've ever read, not just because it goes for the "Everything you've read has been a dream" twist at the end of the story, but also because he retcons that twist all the way to the beginning of the first novel, rendering them both figments. According to the afterword I read (and this was 12 years ago, so it might be a little spotty), the author disliked the negative connotations of the original novel, and wrote this to fix what he didn't like about the story by effectively erasing them both.
What makes it even more disappointing is that the author does a great job of building suspense. In the book, Rosemary's son is grown up, and still the son of the devil, but he's running a worldwide, all-pervasive church. He has plans for something really big, but he won't tell Rosemary what it is, his sincerity is often questioned by Rosemary, and she can't quite bring herself to trust him. See, she's been in a coma for about 20 years, and can only rely on secondhand information to draw her conclusions about her son. So there's a constant "Is he or isn't he?" tension going on throughout the novel, and it's done very, very well.
Unfortunately, we never see how that tension breaks. She wakes up before she can see for sure, and is told that she fell and hit her head at the party in the first novel, knocking her out, where she dreamed the whole thing up. Before reading this book, I had never, EVER been so angry with a book that I wanted to throw it across the room. Since then, I haven't either, but this one was a horrible disappointment.