Each succeeding book in this series manages to feel like it belongs in a completely different genre.
Dollhouse had a very horror-thriller vibe to it, Paper Dolls was a detective novel/abuse survival story, and Marionette changes everything completely once again and manages to make the story into a dystopian science-fiction.
The beginning of the book was a bit jarring, because like the second book from the first, it picks up immediately where the previous one left off. That wasn't such a big deal when I read the second book, because I got it shortly after I'd read the first one. There was a break of a few months between me reading the second and third, though, so it took me a minute or two to remember what the ending of the second book was.
It might just be easier to read all of these one after another - there's no recap at all of the ending of the second one, so you're going to be a bit lost if you don't remember what happened.
Marionette, much like Paper Dolls, doesn't quite manage to live up to the suspense maintained throughout the entire first book. Even though by now we see that the Dollhouse itself was the least of all evils, it still managed to create an atmosphere of horror and helplessness. The castle doesn't quite capture that same atmosphere, and Cassie and Molly have progressed enough as characters that they refuse to be pushed around like they were in the first book. While it's good to see the characters growing, it changes the tone of the story dramatically now that they're actively resisting what's being thrown at them.
Ethan makes a reappearance as a main character in this book, a few chapters even allowing us to see what's going on from his POV. He's essentially there to fill everyone in on the plot, and act as motivation for Cassie's choices at the end of the book. There's also finally a touch of romance thrown in. Not much, but we certainly waited long enough for that kiss!
Passionate kisses aside, this book was actually sort of boring compared to the previous two. Not much happens. Cassie and Molly go to look for the book. They don't find it. Ghosts. The end.