Rebel spring opens right where Falling Kingdoms leaves off. And I will admit that I had forgotten most of what happened in the first book because I read it so long ago, but I will tell you this, Morgan Rhodes does a great job of slowly revealing what happened before, without info-dumping. This is something i always look for in sequels because I think it is very important, especially to someone who does remember most of what happened. You don't want to bore them right off the bat. 
 This second installment was fantastic for many reasons, because it is told from many different perspectives (like Falling Kingdoms) but I thought that this time around there was so much more substance to each character. Especially Magnus, who I seriously hated in the first book, but grew to like in this one. He is such a complicated character and you that in this book. You discover what he has to deal with internally and with the expectations of his heartless father. 
 Celia was another character who I loved in this novel, not because her and Magnus were so connected in this book, but because she grew so much, and we her having to deal with feelings she never thought she would have to feel. It was great to see the evolution of her relationship with Magnus, because it goes from one spectrum to the other over the coarse of the book, and i loved that.