One of my more recent favorite books was "The Historian," by Elizabeth Kostova. It's not light reading, the tome weighs in at a hefty 642 pages. But every single page is worth it- so much so that I've read it twice, which is more than I can say for most of my favorite adult books published after, say, 1980. This Librarian of the Dead has so much stuff to read all of the time, that I took the time to read that TWICE. Think about it.
I loved the way "The Historian" mixed horror and supernatural themes with mystery, adventure, historical fiction, and a rich sense of setting. Specifically, the book explores the legends of vampires and Dracula in relation to the main characters. If any of this has made your inner reader perk up, I highly recommend the book. Don't be intimidated by the size, it moves quickly.
For those who just can't stomach 600+ pages, or have already read "The Historian" and enjoyed it, I would not hesitate to suggest Andrew Pyper's "The Demonologist."