Tl;Dr: Excellent YA time travel book. Time travel was well developed and executed, the romance was not overbearing or obnoxious, and the ending leaves you desperate for more. 
 Firstly I really just loved the premise of this book. I tend to be wary of time travel, because authors tend to fall into one of two traps (if not head first into both)- they either romanticize the past, or fail to implement any real logic into their mechanics "because time travel." Passenger does neither. 
 On the contrary, Passenger pulls exactly zero punches when it comes to societies of the past. The gross mistreatment of people of color and woman remain a constant presence, and Bracken does an excellent job including how that impacts her characters decisions and behaviors. So A+ on not romanticizing really shitty time periods. 
 The logic of the time travel mechanic also remains consistent throughout the book, which is great and all, but the part of this that I found really cool was how the antagonists had used their ability to travel. It is SO accurate to how people would actually use time travel, and I'm kind of astonished this is the first time I've found a story that uses it this way. Passenger definitely brings some ethics into question when it comes to time traveling, and the reality of what has happened and is happening in this world is really crazy to think about. I'm really excited to see how the next book builds on these pieces of the story. 
 Passenger does sort of fall into the "insta love" category a tad, but it is really more of "insta attraction," which is fair, I think. The relationship itself does take most of the book to develop, which also helps keep it believable. I could also just kiss Bracken for not creating any stupid, unnecessary conflict or tension just for the sake of conflict in their relationship; for a long time I thought that's where it was heading, and I was so happy when it steered off that course. After the Selection Series, I just cannot handle that shit anymore, so that definitely earned Passenger points with me. 
 My only real complaint is I think it was missing a sense of urgency from time to time. I often went a few days between reading sessions because I just didn't have the need to know what happened next. It was never boring, and I definitely enjoyed the book, but I was missing the driving desire to know what happened next that keeps me awake reading all night. 
 All in all I really enjoyed it and would definitely recommend it! Sign me up for book 2!