The new novel by Adam Haslett, “Imagine Me Gone,” is a powerful story of a family trying to cope with mental illness. It’s summer in Maine and Alec and his sister Celia are rowing a boat on a lake with their dad. He lies down, closes his eyes and says to them, “Imagine me gone.” They’re supposed to be getting a lesson in how to guide the boat safely to shore on their own in case something happens to him. However, it turns out to be more than a simple lesson in lake safety. It’s a preview for the lives Alec, Celia, and their older brother Michael will soon have to live after their father kills himself.