Here's the thing: you can't blackmail people into friendship. You can't appear out of nowhere and drop your most intimate secrets on a passing acquaintance so that he will consider you his next best friend (and spill his most intimate secrets on you). Most of all, authors can't manipulate readers into liking an obnoxious character just by inventing a very sad story about him so that you can't but pity him. 
 This is what Maggie Stiefvater does with Henry Cheng in this book. 
 After having introduced him as a conceited, childish rich kid in the previous books, now she decides that he has to play an essential role in the story (I personally hate when authors introduce main characters in the last installment of a saga, but maybe it's just me), so she pushes him down our throat using the sad story of poor Henry just wanting to make friends plus a terrible thing happened to him when he was a child so we have to like him, right? 
 It doesn't matter that poor Henry already has his clique of friends. They are not cool enough and he wants to be part of the cool circle. 
 I hate to be emotionally manipulated. This is exactly what Henry Cheng does with Gansey and Gansey, having the personality of a pincushion, obviously falls for it. So when he's finally on the verge of discover what he's been looking for, for 10 years, he doesn't asks for the help of his most intimate friends, who patiently followed him in the search for years. No, Henry (who conveniently appears out of nowhere at the right moment in the right place) is the only person that can help. After all he mixed with Gansey's pompous family way better that Adam and Ronan so let's just throw Adam and Ronan away, right? 
 Besides he's totally unnecessary plot wise, remove him from the story and you would have exactly the same outcome, but with much less irksome commentary on the side. 
 Nonetheless I was going to give 4 stars: the book has some very good moments and it remains gripping till the last chapters, then I reached the end and.. ugh! 
 Stiefvater builds a complicated castle of perils and subplots and then she just stop writing about them. What was the point of Artemius reappearance? And Gwenllian? Why couldn't she say what she knew two books ago? Piper Greenmantel's storyline seems completely meaningless: yes she awakes the demon and then what? This terrifying selling she organizes doesn't lead to anything. Seodenk? Totally unnecessary! The Grey Man? a mere distraction. At some point of the story Noah is totally forgotten ((view spoiler)[The demon attacks Blue through him and nobody feels the need to check on him? (hide spoiler)]) and the author rushes to fix this in the end with another repetitive, meaningless chapter. 
 Plus some editing is missing: in the first chapters there are many repetitions and even some inconsistency (at some point Orphan Girls is not there and then she is; we learn that Gansey knows of Blue's vision and Blue knows that he knows. Wait, when did that happen? someone forgot some chapters somewhere in the middle) . 
 In the end then the resolution of Gansey's fate is so anti-climatic and the epilogue definitely disappointing! Besides, the most emotional scenes weren't emotional at all, Adam and Ronan's in particular felt so rushed! And Adam didn't even need to spare a moment to notice that a year ago he liked a girl and now he likes a boy? I suppose that a teenager boy would need a minute to process this. it is also obscure to me why the author felt the need to dedicate so many chapters in this and the past books to Gansey and Blue's relationship but she's always so superficial when it comes to Adam and Ronan. The basically never talk, which is kind of frustrating. 
 I also found particularly annoying that Blue (basically useless in this book) kept complaining that she wanted "something more" in her life but she never did anything to change this (and it is obvious that she has no idea of what this "more" is). At some point in BLLB we learn that she wants to go to college but she can't because she has no money and her grades are not good. Of course she can't do much about the money but the bad grades? Whose fault are them? Hasn't she ever hear of one Adam Parrish? What I find absurd is that Adam habit to work hard is often depicted as a partially negative attitude: he's prideful and over-achieving. On the other hand Blue, who dwells on her misfortunes and keeps despising her schoolmates for being barbarian rednecks(!), is sweet and mature. I really don't know who is more classist at this point, she or Gansey? 
 ******SPOILER ***** 
 Among the many things that seemed rushed or incongruous in the end: Adam is possessed and then he learns to fight the demon so that he can... bump fists with Gansey? Ronan is dying and then he is not without half a word about it. Ronan's mom? forgotten. If Cabeswater could give part of its life for Gansey's, why coudn't it do the same to kill the demon? Not much sense here plus the scene of Gansey's resurrection is so bland... 
 And that epilogue.... why Adam should want some sort of relationship with a man who made him deaf and admitted in his presence that regrets his birth? And poor Blue &Gansey? don't they deserve and adventure alone? Why should they take Cheng with them as a over-enthusiastic pet? Only Ronan's part was fitting but also that so short and disappointing! 
 A book that in the end didn't live up to my expectations.