I suppose that a major charm of Stiefvater's stories is that in the end you have only understood half of the novel. Or you're missing some kind of details. Because that's how I always feel at the end of Raven Cycle's books. Blue Lily, Lily Blue is full of this confusing magic and tha the same time is one of my favorite novel of the series. 
 So, as always, the author use the particulars of "strange and bizarre", making thing up, complicating the events, and yet at the same time it focus so much on the characters that I had the constant feel, especially at the end, that something was missing. 
 The plot develop in something really delicate, that I feel how is almost impossible to talk about it without doing a little bit of spoilers. For many times Stiefvater depicts a little fact, a word, or a phrase that seem useless at the start but in the end they all compose a good system of events. And it also leads to new characters! 
 I had a bit of hard times with decisions made by some minor characters, just like Maura's path during the novel, but I definitely fell in love with some new people - that hoped to see more in The Raven King but it never happened. Cry with me, please. 
 In that case I don't feel like my connection with the Raven Boys made a big progress, but I definitely fell in love with Blue. But I should totally note that the mechanic between the crew was great to read about. Everyone complete everyone, even if I think that would have been nice to see more interaction between Ronan and Blue. 
 Okay, talking about the girl, there's a reason why his book is named after her. I couldn't have been more happy discovering how this novel started to move around her and also the discovery of something really important about her family. 
 For reasons, some more close some more afar, I really related to Blue on various levels. How she fights for her future, how she defended herself against bullies - even if it wasn't directly showed - and how she decided to start move towards the truth. I would have loved to have her same strength and same occasion. 
 I also loved the low-toned magic, yet incredibly dreamy, and how it mixed perfectly with the story and the whole character driven plot. Also, Sitefvater's managed to mix that magic with new characters, as I cited above, and one in particular made me really happy... even if he had such little pages. 
 In the end, I think that Blue Lily, Lily Blue is one of my favorite and more complete novel of the cycle after The Dream Thieves, and also the novel where things start to become heavy and serious. Definitely the book you should read if you're not s interested in the series or thinking to drop it.