If you want to read this novel (or should I say 'novella') I would suggest keeping your expectations low. 

A year or two ago I started getting back into reading and one of the books that I picked up was A Court of Thorns and Roses, I didn't absolutely love the book but I thought it was enjoyable. So, having already bought A Court of Mist and Fury  I read that too. I fell head over heels in love with that book, I devoured it. I recommended it to all my friends, I immediately pre-ordered A Court of Wings and Ruin and read that the week it came out. Now, two years later, I'm a completely different reader. I used to fly through books and just enjoy them without thinking about things such as 'character development.' I was reading purely for enjoyment. But the tides have turned friends. I am now more of what you could call 'a critical reader' and can no longer read a book without reviewing it in my mind. This made reading A Court of Frost and Starlight a really interesting experience.
 
I have to say it: This book was a trainwreck. Did I enjoy it? Yes. This series will always have a special place in my heart, but oh man... It's really not that good. It's pretty gosh darn bad.
 
(Almost nothing actually happens in this novella, so there's not really anything to spoil. But if you want to read this without knowing anything about the story, I would stop reading this review now.)

Sarah J. Maas does this really interesting thing where she just completely changes characters without any good reason or foreshadowing. Do we remember the scene at the end of A Court of Wings and Ruin where Cassian and Nesta almost die together? You know, where Cassian says: 

"I have no regrets in life, but this. That we did not have time. That I did not have time with you, Nesta. I will find you again in the next world-- the next life. And we will have that time. I promise." 

Remember that? Well, apparently Cassian's promises don't mean much because in A Court of Frost and Starlight they're not talking. Why? Nesta doesn't want to see him. All the little character development Nesta got in A Court of Wings and Ruin was done for nothing because Sarah J Maas turned her right back into the stony cold bitch we met in A Court of Thorns and Roses. It's messy writing. It's tiring. 
(We all know she's just dragging this out for dramatic tension so she can build a 'hate to love' relationship in the spin-off series. There are a lot of readers who ship Cassian and Nesta and she knows it. She's dangling the ship in front of their noses so they'll keep reading.)

Remember when Tamlin saved Rhysand's life and said: "Be happy, Feyre." Do we remember that? Oh well, you can forget all about it, because Sarah J. Maas did it again folks. All of Tamlin's character development is erased in two seconds and we're all supposed to hate him again. I think she just realized there was no conflict or villain in this story, so she turned Tamlin into our villain. Again. Damn such an exciting and interesting choice.

It's also baffling how little personality personality Rhysand and Feyre have besides constantly wanting to have sex with each other. They used to have a compelling relationship, but everything went down the drain. Sarah J. Maas' smut has always been kind of cringy and questionable (shaking mountains, growling males, you know, casual stuff) but she really outdid herself in A Court of Frost and Starlight.

I dare you to read this without cringing:

"Do you want to begin at the wall, or finish there?" His words were guttural, barely recognizable, and the gleam in his eyes turned into something predatory. He slid a hand down the front of my torso in brazen possessiveness. "Or shall it be the wall the entire time?"
My knees buckled, and I found myself beyond words. Beyond anything but him.

I would give you a minute to process that, but this is a Goodreads book review and I'm not able to do that. 
Let's unpack that shall we? 
 The gleam in his eyes turned into something 'predatory'
 'brazen possessiveness'
Why is this presented as romantic? Are we actually calling our main love interest, one of our main characters... a predator? I know she's not actually calling him a predator but she's describing the look in his eyes as predatory. When he's looking at his lover.... Feyre... girl start running. 
(Remember guys, Tamlin is the worst male ever but Rhysand is an angel, he's such a good male. We love him so much.)

Also.. your definition of dirty talk is just naming the location where you're going to have sex? And her knees buckled because of that? Are high fae just that easy to arouse? 
What's the next scene going to be like? Is he going to start talking about floors until Feyre faints because she's just too damn turned on? I'm confused.

This book is really fun to laugh at. Some things that happen are just too damn ridiculous. I mean.. Feyre paints Rhysand as one of her French girls.... how am I supposed to keep a straight face whilst reading that? 

Lightning round of my final opinions because I really want to sleep, I'm tired.
Here we go:
Sarah J. Maas needs to hire a better editor who actually fixes her messy writing.
The storylines that actually have potential are mostly ignored which sucks. (For example: Elain... why do we still know nothing about her powers? Also: her and Azriel? Why did we see nothing of that? They said like two words to each other in this whole novella.)
Nesta is getting dragged through the mud. 
Feyre and Rhysand are too busy having sex. 
Amren is in a relationship with a male who we know nothing about. We know his name and where he comes from, that's it. Not even one hobby. He says nothing, he just stands next to Amren looking pretty. Boring. 
How many pages can you spend shopping? It felt like that was all Feyre did when she wasn't thinking about painting or sleeping with Rhysand.
Cassian deserves better. He's one of the few characters with a really strong personality. I think he'll always be my favourite character in this series.

That's it. Bye y'all.