The Roads to Modernity: The British, French, and American Enlightenments, by Gertrude Himmelfarb

This book does a great job comparing and contrasting the differences of the Enlightenment as experienced in Britain, the U.S., and France.  It's like an introductory, comparative survey course of the various Enlightenment thinkers in the three countries, so do not expect in-depth or detailed analysis on any particular person.  This book is more to give you a general picture of the intellectual climate of those times.  The treatment of the American Enlightenment is the briefest of the three, which was fine with me because I am more familiar with the American founding fathers and the thinkers that influenced them than I am with French or British Enlightenment figures.

Caution:  Your to-read list will grow as a result of reading this book.