In the US, juries decide facts, judges interpret the law

A good book which analyses the contemporary and historical use of juries in the context of an overall critique of the contemporary criminal justice system is The Collapse of American Criminal Justice by the late William Stuntz.

There are undoubtedly better books describing the history of the jury system, but according to the thesis of the book various changes in the the criminal system in the United States has turned juries into rubber stamps. Jury conviction rates are significantly higher (double or more) today than a hundred years ago. Juries used to have far more discretion than they do today. Now the laws are so detailed, and their definitions so all-encompassing, that juries are given very little leeway to show leniency or to provide the defendant with any true benefit of a doubt.