What do police officers read for relaxation?

I teach science and have an opportunity to help design a curriculum intended to help local officers obtain regular professional development in a college environment. The hope is that this will lead some to eventually earn a degree. The jump to college level content presents real challenges to most high school graduates. Likewise, many of our police despite years of experience have minimal college level training. We expect a wide spectrum of reading and writing proficiencies but want to set reasonable standards and expectations. Hence the question.