Two books that helped me to understand Roger Williams and his views on the church and the state and how they should (or shouldn't) relate to one another are:
Roger Williams by Edwin S. Gaustad (2005)
At 132 pages, this book is a perfect introduction to Roger Williams. It outlines his European roots, time in the Massachusetts Bay Colony, his interactions with the Native Americans and his founding of Rhode Island. The book also includes plenty of primary source excerpts for those looking for the first hand accounts and impressions of Roger Williams.
Roger Williams: The Church and the State by Edmund S. Morgan (1967)
Though older, Morgan's book remains a very helpful resource for those trying understand the maturity and development of Roger Williams' views. This book doesn't spend a lot of time covering Williams' biography. This book focuses on his thoughts and philosophy regarding the tenuous relationship between the church and the state.