I had a strange relationship with this final book in the series.
Mostly, I just wanted it to be done. It felt to me that Stiefvater was told to write a full-length novel when she really wanted to write a novella as the last book. There were too many scenes about the same thing. Oh, Cabeswater is in trouble? Golly, I couldn't tell. You already wrote 5 scenes about Cabeswater going through the wringer in this book, and one more isn't telling me anything more. Gosh, is Gansey upset about his approaching death? Holy cats, who wouldn't be?! Didn't you already tell us this at least a dozen times since the book started? It got sort of... dull over time.
Also, did she change up the narrative without a word of warning regarding Ronan and Adam? Have I been oblivious for 3 books? Does this bother anyone else? I like that she went in this direction, but it is not fully fleshed out, and shouldn't be just an add-on to the whole story.
And one last thing - why does Henry come in as a character only in the last book? He's appropriately sarcastically funny and does seem like a "knight" to Gansey's "king", but does he have to be this naive about the storyline? Wouldn't it have been better to include him earlier so that he isn't quite so much of a third wheel? I felt kind of bad for him during the last chapters.
I was, of course, hugely pleased with her intent in the ending, and I enjoyed Stiefvater's words as much as I normally do, but I wish I hadn't left this series with such a feeling of disappointment overall.