When you're the president's pet, who walks you, empties your litter box or scrapes off your perch? Readers who hunger for information about the nonhumans who've lived at the White House over the years won't learn the answer to those questions, but they will discover that all our chief executives but one, from Washington to Obama, have owned a variety of pets--and, in some cases, been owned by them. In addition to the familiar dogs, cats and birds,...