There's a very good book, Public Enemies: America's Greatest Crime Wave and the Birth of the FBI by Bryan Burroughs, that I recommend. It's the best single book on the subject, and wades through all of the legendary hyperbole about those criminals and gangs to get at the actual truth. Ma Barker, Machine Gun Kelly, Baby Face Nelson, John Dillinger, and Bonnie & Clyde all get covered. The Dillinger sections served as the basis for the 2009 movie Public Enemies with Johnny Depp as Dillinger, but the book was much better.
I have that book. Well written and detailed. I think of all the well known 1930s
criminals, Ma Barker was the one most hyped up, her actual story being much
exaggerated. The caption to her photo contains the description I paraphrased
before. It actually is "A cranky old hillbilly consumed with jigsaw puzzles,..." The
only thing I didn't like about the book is that it is arranged chronologically and
so it jumps from one public enemy to another and then back again, making it
a bit hard to follow.