The last book in the Hex Hall trilogy starts pretty much where Demonglass (review here) left off: Sophie coming out of the Itineris in search of her mom and the Brannicks, a group of Amazonian-esque redheads who want to destroy people like Sophie--i.e., demons, witches, werewolves, fairies, etc. With her best friend, dad, fiance, and boyfriend all possibly dead, Sophie has to figure out a way to stop the Casnoffs from turning the rest of her high school into a demon army.
That plot sounds like bit of a potboiler, doesn't it? Actually, Spell Bound is a very light, fun read with a lot of humor. I think it's a successful conclusion to the Hex Hall series, although not as good as Demonglass. Sophie is always quick with the one-liners, and that gives Spell Bound some much-needed humor (my personal favorite was, "Jackass, jackass on the wall, where's the info on Hex Hall?").
That being said, it doesn't feel like a lot goes on in Spell Bound. For most of the book Sophie is recovering from her adventures in Demonglass and trying to figure out how to defeat the Casnoffs. There is a lot of build-up to the final battle, but the solution feels too easy and pat. I also have issues with the events that happened in the last fifty pages--again, it was all just too convenient. There wasn't really a sense of emotional resonance at the conclusion of the series that I would have liked.
Overall, though, I think Hex Hall has been a pretty successful YA trilogy. I had some doubts when I read the first book (review here), and there are things in the plot that are very cheesy and predictable; but Rachel Hawkins manages to pull it off with great writing that makes the novels fun and enjoyable.