Roger Williams and the Creation of the American Soul: Church, State, and the Birth of Liberty by John M. Barry came out in 2012 and is highly readable. As the title suggests it focuses mostly on William's contributions of religious liberty, though Barry perhaps slightly overstates William's importance to later thinkers on the topic.

There are older biographies by Edmund Morgan and Edwin S. Gaustad which would be worth consulting, the titles are Roger Williams: The Church and the State and Roger Williams respectively. Both scholars were well reputed historians. These books are fairly short, so they would be a comparatively quick read.