A good place to start...

...when learning about Catholicism, religious liberty, and American politics is Render Unto Caesar, by Philadelphia's new Archbishop Charles Chaput, O.F.M.Cap., formerly of Denver.

Clearly written and fair, Abp. Chaput gives a good overview of the history of American religious liberty, and calling upon Catholics to live their faith in a public and open way, to live by their principles and to defend their traditions in the face of secularizing forces.

I read this as a beginning of my upcoming work on Kulturkampf and Ordered Liberty, and recommend it highly as a general primer on the topic.