Alina's character has made such a transformation from herself in Shadow and Bone. Even The Darkling makes note of it when he makes a sex joke and she dishes it right back when two novels ago she would have blushed and looked away. Speaking of The Darkling - we learn is name! You know Leigh definitely is definitely going to tie up all loose ends when she reveals even that smallest detail.
There are so few Grisha left for Alina after the events of Siege and Storm, but this small group (we're talking 12 people) ban together in such an amazing way to stand against The Darkling and his cruelty. Considering the number of characters Leigh has to work with, each and every one of them stand on their own two feet. Their personalities never blend into one another and they don't come across as one secondary blob. A true testament to Leigh's writing ability.
It's not just characters though. She does an amazing job with descriptions, scenery, Alina's internal thoughts. Nothing about Ruin and Rising comes across as a 417 page novel. You can't escape her beautiful writing and the real world doesn't bleed through while you're reading either.
I'm not really going to touch on the romance, there is such a huge divide in this series that I know people will be pissed, pleased, and contented. Not many people are able to pull having more than one love interest, but it's real and logical in The Grisha. There is no "clear choice" or a guy thrown in there just to make things interesting. Leigh did a wonderful job with her love quadrangle, simple as that.
To boil this rather long review down, Ruin and Rising is a beautiful ending to a beautiful series. The writing is fantastic, the continuously developing characters lovable, and the plot simply amazing. I was already a fan of Leigh's halfway through Shadow and Bone, but knowing that she can end a series just as well as she can start a series has sky rocketed her to a position on my top authors.