Oh good lord.
One of the things I really like about Joe Hill, besides him being a really good writer, is that he's Stephen King's son and he's not making a big deal out of it. Totally downplaying it. With "A New Novel by the Son of Stephen King" on his covers he would sell a billion jillion books no matter how good they were, but he didn't do that; he's become a popular author on his own merits. Aliza Kellerman has indeed done that. She "co-wrote" this book with her mother, Faye Kellerman, whose books end up on the New York Times bestseller lists and on the scrawled lists of little old ladies who keep their holds maxed out at the library.
I've never read a Faye Kellerman book but I'm going to hazard a guess that this book was mostly Aliza and only a smidgen of Faye. Aliza is a junior in high school and this book honestly reads like a fiction version of the essays I and all my classmates wrote at 16 and 17. Especially the creative classmates. The writing is awkward. The dialogue is stunted and slow. The book is extremely repetitive, as characters explain certain events to other characters over and over. It was like one of those kid's shows, only a bit more fleshed out. Not a direct quote, but it was like reading "We fell down in a cave. We fell down in a cave and got hurt. Oh, no. There is no medicine, after we fell down in a cave." over and over.
Not to mention the giant gaping plot holes that come from having two universes that are exactly the same save that one does not have medical science. There is quite simply no way that the world would be the same. I might hazard a guess that if humanity managed to evolve out of the medieval era without medicine, by 2010 they would all be dead or certainly never have expanded. There's a lot of silly wordplay like "spills" for "pills" (why?), absolutely flat characters, and "romance" that consists of shoving the two main female characters with the two main male characters.
Ok. I'm done with this.