I kept questioning whether or not I liked this book as I was reading it. I think I probably won't read any sequels. There were definitely things I liked, but also things I didn't. 
 Pros: 
 -They actually address the ethics of travelling through dimentions. 
 -Marguerite's period is a thing they actually addressed. Normally I don't care about this, but in dystopian or fantasy books that take place over long periods of time, in unfamiliar environments, I sometimes wonder what the girls are doing about it (coughKatnissintheareacough). I like that this was addressed. 
 -Good use of foreshadowing. There's a little bit of spoilers here, but Marguerite's repeated mentioning of her father's love of the Beatles was excellent. It seemed like it was just being used as a way to show the differences between universes, but it had greater significance. 
 -Similarly the foreshadowing about Theo. 
 -Also explaining why Marguerite didn't need reminders. 
 -I had wondered why the book took so long to explain what happened in your own universe when you started traveling through universes, but they did explain it, and it made sense why they waited so long. 
 Cons: 
 -The relationship between Marguerite and Paul felt rushed. I felt like they were put together in a relationship because Gray thought she needed to have a romance to attract readers. 
 -What happened to her Dad's firebird? He travelled to the waterworld, then lost it? 
 -While it made sense in the story why they were stuck in the Russian Princess world for so long, it really stalled the story to have her stuck there. 
 -Not really a con, but I would have found it interesting if Marguerite was called something different in some of the different universes. Like, if she was called Josie in the Russian Princess universe (since she was the eldest daughter there).