Simple Google searches can often get you all the background information you need, but think about using some of Google's special features if you need to get particular perspectives or treatments of your topic.
And note - with Google books, the full text of a book, or much of it, is often there for you to read online. This can be invaluable when you need book resources and you don't have time to get to the library to find the book on the shelf. See below for more!
When you perform a search in Google Books, you can usually do a second search in the full text of the books you retrieved. In the example below, the search
"freedom of religion in the united states"
retrieved the book, Constitutional debates on freedom of religion: a documentary history, by John J. Patrick and Gerald P. Long. Doing a search for "Roger Williams" in that book retrieved the following results. Give it a try!