I have become very wary of YA trilogies these days. I fear the third book. Into The Still Blue is not a final installment to be afraid of. It wrapped up the series nicely. I am very satisfied with the ending.
Into the Still Blue begins shortly after the ending of Through the Ever Night. If you need a recap, the bloggers over at Recaptains have recaps available for both books in the series. (http://recaptains.blogspot.com/search/label/Veronica%20Rossi%3A%20Under%20the%20Never%20Sky) What I liked most about this story is that is has both action scenes and slower character scenes. I like books with a lot of action however there was one book I started last year that I had to stop reading and not finish because every time our characters finished one obstacle another one started. There was nothing but action. I need my character to grow, develop and not constantly be fighting for their lives. Veronica Rossi accomplish this for me in the final book of this series.
Another great thing about this book is we get to learn more about some of the other characters in the book. Soren is one that stuck out for me the most. His character development from book 1 to book 3 is remarkable.
I'm not going to give a lot of plot details because it is a third book and I feel like you should get to discover it on your own. I do recommend you reading it as I think it ties up nicely in the end. The ending (like the very last page or two) were a little lack luster for me but i thought it was a nice ending overall.