I nominate "SoulStorm" by Chet Williamson.
Three men are offered a million dollars each if they will spend a month in an isolated Pennsylvania mansion, the Pines. There they will confront madness, murder and the ultimate evil....so that their billionaire host might find the key to life beyond the grave. But as they learn, dead souls dwell in the Pines. And death is just the beginning....
Available from the Kindle Store for $2.99