Through the Ever Night earned three of five stars from me only because the writing is pretty good and I already knew and liked the characters and I've met the author and she's a nice person and honestly, that flavors my feelings about books. If I'd picked this up without having read Under the Never Sky I would've had to think hard about even giving it one or two stars.
Through the Ever Night starts a few months after the end of Under the Never Sky. There was quite a lot of action in the first book and then there was practically no action in this one. Aria has found out that her mom died and is still Outside. In the first chapter she hooks up with Perry and hangs out with his tribe for a little while. Things are pretty tense, because the Tides (his tribe) don't trust or like Dwellers and they see Aria as a Dweller. Cinder is still living with the Tides too, and still no one quite knows who he is or where he came from or why he can do what he does. So of course the plan is still on for Aria and Perry to go looking for the Still Blue, and they do make a little progress on that in Through the Ever Night, but they don't make much. And they don't get there. (Hope that's not too much of a spoiler. You do know there's a third book; they can't resolve stuff in this one.)
I got hooked with Under the Never Sky and I do plan to read Into the Still Blue to find out how it all ends, but this second book just felt like filler to me. This might have been better as a duo of books. I recommend this to people who enjoyed Under the Never Sky.