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.

On Looking Into the Abyss: Untimely Thoughts on Culture and Society, by Gertrude Himmelfarb

This is an excellent series of essays on various thinkers and philosophical ideas that have filtered out of academia and become influential in the modern world to the detriment of clear and logical thinking based on objective truth.

Her essay on John Stuart Mill, <i>Liberty: One Simple Principle</i> is especially good as an introduction to Mill's work, particularly <i>On Liberty</i> and the effects it has had on how people define "liberty" today.  Also particularly enjoyable is the essay on footnotes <i>Where Have All The Footnotes Gone?</i>, which suggests that their increasing absence in scholarly works, and scholars' increasing insistence on dispensing with them, is a symptom of the blurring of the line between history and fiction, footnotes being one of the ways a scholar's work can be checked by readers.