Okay, I'll see if I can sum up the story so far, I'm on chapter 13 now~
Spoiler
Yuuko is a ghost with amnesia. She doesn't remember when, how, or why she died. Niiya can see her and he helps her solve her mysterious death. There are seven mysteries that are based on Yuuko and each one of them tell an aspect of her death. Later, apparently another girl can see her and says that she is an evil spirit that is "spiriting" Niiya away, he doesn't believe her and they go to solve the mystery. Yuuko isn't spiriting him away and that leaves the mystery of who is the ghost that the new girl, Kirie, can see... some cases come up and they get solved and romance starts building and a new ghost appears, which is where I'm at now...
It's pretty nice and there are more details to it then I went into... I would recommend it~