The Sorority Trilogy by Tamara Thorne
Ok, started this one just to see if it might be usable in a BookObsessed series swap. I had no idea if the books were any good or not. I had never heard of them before I got them, nor heard of the author. They had been sitting on my bookshelf for months. I thought I'd read 50 or so pages of the first one just to see and I sped right through all three. Three books in one week. That is a new record for me.
The books built on each other. They were a mixture of ghost story, wizards, Arthurian history and horror. The story was really intense and kept me interested (obviously). A very pleasant surprise.