The Golden Enclaves
What a fantastic conclusion to the series! What I was afraid was originally an amusing YA Hungry Potter Games turned out to be a really interesting moral story about the development of a young woman's abilities and ethics. With a global warming allegory at that.
I'm not going to rehash the plot here, but I will say it was very solid. Structurally this book is quite different from the first two, now that school is over, and the author managed to find a new plot framework that worked very well. Mostly though I appreciated El's transition in her ethics, understanding what her role would be in the world and resolving the contradictions of both her own abilities and the nature of magic.
There's a huge twist / development in this book that explains and reshapes a lot of the concepts and narratives in the first books. I gather this has been controversial with fans but I think it worked perfectly. Also I'm not sure but there's several hints that the author had this planned from the very first book; not in detail but in broad strokes. It definitely felt like it all made sense to me, not a deus ex machina at all. Smart writing and quite satisfying.
My one quibble is I think Orion got short shrift in this book, barely a character at all and off-screen for most of the action. Instead several of El's friends are the other characters but I had a hard time caring much about their stories. Also there's a bit of romance in this book that just feels out of place. It was charmingly awkward in the first two, here it maybe should have just been elided.
But minor complaints aside it's a great book and really justifies the whole trilogy. I also recently read the author's Uprooted and liked it quite a bit as well, if you're looking for more.