In a book about military science, you may be surprised to find passages like, “I’m interested in diarrhea as a threat to national security.” Or “On top of its other charms, the maggot breathes through its ass.” Or “If you don’t have a pair of cadaver shoes, you’re not doing enough research.” You’d be surprised, until you realized that this book on military science is also a Mary Roach book, the science writer behind books such as “Bonk: The Curious Coupling of Science and Sex” and “Stiff: The Curious Life of Human Cadavers.” Her new book is called “Grunt: The Curious Science of Humans at War.”