The Dead Whisper On, by T.L. Hines.
This is Hines' second novel. Though not a sequel to Waking Lazarus, it plows similar material: creepy events, eerie spirituality, and an interesting Christian take on the ghost story.
Canada MacHugh, a hard-working woman in Montana, hears the voice of her father, dead for 11 years. And Dad has a mission for her: helping him and other (supposedly) dead folk go about and perform missions of mercy and goodness. What could be wrong with that?
Well, of course, plenty can go wrong with that. And it does go wrong for Canada, and she is thrown into a variety of dangerous situations, all the while trying to figure out exactly what the ghosts are.
Hines' books have a dry sense of humor that counteract the suspense/horror aspects well. No book is everyone's cup of tea, but this one is a solid example of the spiritual thriller genre.