I recommend Through the Ever Night. 
 ...done and done. :D Veronica Rossi messed me up big time. 
 While it was not as stunning as I thought it would be (based on the glowing reviews I saw), I think it is one of the best middle books to ever come out in a YA dystopian series. Rossi strengthened the characters, developed more the (previously) vague dystopian world in Under the Never Sky, and while the loss was not minimal, Rossi left hope and joy that are enough to leave readers (almost) fully compensated. 
 If I were to base my rating on the story advancement alone, I'd say it was really good, maybe even deserving a 5-star rating if I was being generous. But... I was so emotionally invested with the characters that I can't help but get cranky if things don't go Aria's way, or Perry's. 
 From being a Dweller to becoming a warrior, Aria was intimidating. I didn't expect her to be this ready for her next goal in the Outside. She was no longer a burden. She was now Perry's equal. I really liked her character's development. Aria came out mature, responsible, and not self-centered anymore, compared to the first installment. Oh, and she's not whiny anymore, hooray! 
 It's hard not to judge Perry on his decisions because it was no longer about him and Aria alone. There's the tribe to think about, the supporters of Vale who has not yet fully accepted Perry, the worsening Aether that's threatening to obliterate his home. But. but. I wanted more display of his hunting and fighting skills. and I just wished he got to spend more time with Aria in this sequel, that's all. I adored Roar, but he's not Perry for Aria. you know? haha! 
 Sable's character reminded me of De Malo from Rebel Heart, at first. and I actually liked Soren! I found him funny. But I cannot wait for him to try getting back at Perry. He's in for a surprise. Pound him to pieces, Savage! 
 Through the Ever Night will take you to a roller coaster ride. The romance angle is played very well, and the dangerous world of Aria and Perry will leave you an unsatiable taste of their desire to survive. 
 Cinder! 
 4.5/5 stars 
 *** 
 I am so going to read you in one day. well, today. or maybe in six hours? four? 
 And I was mistaken, because yeah, that's Perry on the cover. *beams happily* 
 *** 
 That is so not Perry. Where's my blond-haired warrior?!!! So, I think that's Roar in the cover.