Fire and Light by James Burns provides a compelling overview of Enlightenment thought, as well as how it influenced the American Revolution and its leaders. It does acknowledge its own positive bias towards the period, considering that Burns sees it as a good thing. However, the Very Short Introduction series from Oxford has a book on the Enlightenment that takes a more questioning look at the subject. It's important to note that a lot of introductory history about the Enlightenment casts it in a positive light, but there's a lot of interesting counter-Enlightenment scholarship to get into as well, after the happy stuff of course. For that, I recommend starting with Immanuel Kant's short essay "What is Enlightenment?" and immediately following it up with the introduction (at the very least) of Justin Smith's book Irrationality.