Books in the Ayakashi Triangle series

Matsuri Kazamaki's job as an exorcist ninja means that his livelihood depends on driving away the ayakashi spirits who endanger humans. He's been protecting his childhood friend, Suzu Kanade, from the shadows for years, as she effortlessly attracts ayakashi. What will happen when Suzu encounters a creature who looks like a simple cat, but who is really Shirogane, the king of ayakashi?