As an individual book, it's not very good. The plot is thin, most of it centered around Nesta & Cassian and the terrible reality that is living with PTSD. 

The smut, while not always realistic, is actually not half bad. Idk why she doesn't just write erotica and be done with it. 

As part of a series, this book suffers tremendously, and does the original 3 books a disservice in sacrificing plot and characters for an exciting romance to entice young readers. 

J. Maas' writing has not improved from the first book. The prose is repetitive and tired, the grammar mistakes glaring. The characterisation is especially poor; everyone has the same vocabulary and habits.

As far as lore goes, there isn't any. This series is a far cry from The Modern Faerie Tales. There are brief mentions of suriels, kelpies, and nixies, but that's as close to real lore as it gets. Fae have taken the place of vampires and werewolves for which mysterious / potentially dangerous creature young girls want to read about and therefore the faeries in this book are just kinda magical humans lol. They would have made better witches.

It's clear that J. Maas wants to keep this world going for as long as she can. The fourth book in the series is basically a slice of life fanfic devoid of any plot (though it is a cozy read), and this book could would have made a much better spin off. 

There's some cool stuff with the girls fighting and the Dread Trove, but the rules of this universe grow increasingly ridiculous, with a made up on the spot kind of vibe. J. Maas favors writing a specific kind of romance and smut, which is fine, I guess, but she tends to lose the thread of the actual plot in order to cater to a spicy romance, which should be a subplot and not the main focus. This mistake shows itself over and over, but never moreso than with Feyre's pregnancy. Rhysand respects her right to choose in every situation except if she's carrying his child? And Feyre, who shut down an entire kingdom in revenge for such treatment, is just totally chill about it? Makes no sense. It also made no sense that she got pregnant at all, with how careful everyone is with the danger of war on the horizon. I mean, what?

One of the main plot points is learning to live with PTSD and not let it consume you, which admittedly is great, but it would have made more sense for this book to be split between Feyre, Nesta, and Elain, with the sisters finally coming together like they always should have been. It would have also given the readers a chance to get some insight on Elain/Lucien/Azriel, and Feyre more of a storyline. 

But I digress; the story has some good bones. All the war and riddle shite is great. I'll probably read the next one, too, but people act like this series if everything and frankly- while it's very entertaining, like bad fanfiction-the writing is clumsy and reeks of bad planning.