Floating staircase / Ronald Malfi.

Enhanced descriptions from Syndetics:

Following the success of his latest novel, Travis Glasgow and his wife Jodie buy their first house in the seemingly idyllic western Maryland town of Westlake. At first, everything is picture perfect--from the beautiful lake behind the house to the rebirth of the friendship between Travis and his brother, Adam, who lives nearby. Travis also begins to overcome the darkness of his childhood and the guilt he's harbored since his younger brother's death--a tragic drowning veiled in mystery that has plagued Travis since he was 13. Soon, though, the new house begins to lose its allure. Strange noises wake Travis at night, and his dreams are plagued by ghosts. Barely glimpsed shapes flit through the darkened hallways, but strangest of all is the bizarre set of wooden stairs that rises cryptically out of the lake behind the house. Travis becomes drawn to the structure, but the more he investigates, the more he uncovers the house's violent and tragic past, and themore he learns that some secrets cannot be buried forever.