It's pretty rare that I want to read the next book in a series right away, but Stiefvater is clearly a master of series writing. I'm only familiar with her solo book, The Scorpio Races, and I picked up this series on the strength of that writing. Now I better be careful or I'll scarf down all 4 in a row.
Stiefvater answers just enough of the mystery to make the ending of this first book palatable, and leaves just the right kinds of threads dangling. What in the world will happen to Adam? What is this mysterious connection between Gansey and Blue? What really occurs when you "disappear" someone by mistake? As in The Scorpio Races, I adore how she creates something that is magical but completely rooted in our existing world, so that you have to work a little to understand where the magic is occurring in the story and when you get your nose out of the book you still feel like you're in there.
I was surprised that she is also able to write using a different language and style. This world is a current one, in which people act like teenagers, wear usual teenage clothes, and live (mostly) regular teenage lives. It's not rooted in a culture of far away and super traditional. Of course, I loved them both, and I'll be sitting on my hands not to jump into the next book right away.