In the late 1980s in the UK, a border collie-Doberman mix began paying extreme attention to a mole on his owner’s left thigh. The 44-year-old woman noticed that the dog would sniff there for minutes on end, trying to bite it off whenever she wore shorts. The dog’s fixation wasn’t random. What the dog was smelling wasn’t just the mole, but the melanoma causing it — and its subsequent removal might just have saved the woman’s life. In this new book, author Horowitz shows how canines, whose sense of smell is anywhere between 10,000 and 100,000 times more acute than...