Finally tracked down and killed themselves on May 23, 1934, Bonnie and Clyde remained all but forgotten, relegated to pulp magazines and a B movie or two, for 30 years. The infamy they enjoy today can be traced almost exclusively to the wonderfully filmed, if thematically wrongheaded, 1967 movie “Bonnie and Clyde,” a paean to hippie-era themes of anti-authoritarianism and youth rebellion. Before the movie, there had been precisely one substantial biography. In the last nine years alone, by my count, there have been 10. And now, in time for the 75th anniversary of the pair’s deaths on a Louisiana road, come 11 and 12. The one to pick up is “Go Down Together: The True, Untold Story of Bonnie and Clyde,” by Jeff Guinn, which is easily readable and includes much of the last two decades’ new scholarship. The one to avoid is “Bonnie and Clyde: The Lives Behind the Legend,” by Paul Schneider, a book whose idiosyncrasies include the author’s devotion to such italicized gun sounds as...