For one thing, it is over-long. It contains a lot of information, with abundant quotations from Enlightenment thinkers. This makes for authenticity, because these people are allowed to speak in their own voices (although usually in translation), but there are so many citations that it becomes difficult to identify a continuous narrative thread. I found myself being constantly tempted to skip the quotations to find out what Pagden himself made of it all. Eventually I resorted to reading the introduction and conclusion in an attempt to get a better grasp of the whole.
Even here, however, I encountered difficulties. The conclusion, where I had hoped to find a clear summary of the author's views, instead wanders diffusely among a number of themes, some of which are of uncertain relevance to the main part of the text. Pagden devotes a perhaps excessive amount of space to discussing how far the Enlightenment can be held responsible for the excesses of the French Revolution. Then we get a counterfactual in which Pagden speculates that if the Protestant Reformation had never happened the result might have been the conquest of Western Europe by the Ottoman Turks and the establishment of Islam as the state religion. I find some difficulties with this speculation.
First, the Reformation did not happen within the Enlightenment period, although it can perhaps be seen as paving the way for the Enlightenment. But that aside, Pagden himself concedes the questionable plausibility of his imaginary scenario but explains that he introduced it to point out that something similar did indeed happen in the Islamic world. A remarkable period of scientific advance and philosophical speculation, inspired by the Greeks, occurred in the early Middle Ages but was followed, after the twelfth century, by intellectual stagnation and the imposition of strict religious orthodoxy. As a result, Islam became isolated under the Ottomans and its economic, scientific, and cultural development largely ceased.
Here, I think, Pagden starts a hare that should have been left peacefully asleep. The question of why Islam fell behind the West in the modern era is a huge one and should not have been introduced in a single paragraph in a concluding chapter. The causes of the relative decline of Muslim science after the twelfth century are more complex than Pagden implies and cannot be simply blamed on religion. For a good discussion of this, see An Illusion of Harmony: Science and Religion in Islam, by Taner Edis.
Pagden's main focus is on philosophy, politics, and religion. There is relatively little on science, which I regretted. For me, the best chapter in the book was the third, "The Fatherless World", where Pagden describes how Enlightenment thinkers made telling criticisms of religion. He has no doubt that some of them, including Hume, were full-blown atheists, although for reasons of prudence hardly any admitted this openly. It becomes pretty clear that Pagden's sympathies lie with the sceptics, which perhaps makes the discussion a little one-sided.
Chapter 5, "Discovering Man in Nature", is also good. It has a lot of interesting and often amusing material concerning the eighteenth-century writers' views of the peoples encountered in the south Pacific and elsewhere. They tended to see the inhabitants of these places as exemplifying the Noble Savage, man in his natural state unspoilt by religion, but deep misunderstandings existed on both sides.
A disconcerting feature of the book is how bad the proofreading has been. So we get, for example, "though" instead of "through", "dammed" instead of "damned", "ensure" instead of "assure". More seriously, a date given as 1807 is nonsensical in the context and one is forced to guess that it ought to be 1817. Errors of this kind are increasingly common everywhere these days but they are unexpected for an academic publisher like OUP.
Typos aside, the text itself shows signs of carelessness. On p.28 we read that the Peace of Westphalia, which ended the Thirty Years War, "was the first treaty between sovereign nations which succeeded in creating a lasting peace and not merely a temporary ceasefire, as all previous treaties had done." But the very next paragraph begins: "The treaty did not achieve an immediate or, in the end, a lasting peace." As we read on we see what Pagden meant to say: fighting did go on but it was no longer religiously motivated. But the reader should not be expected to mentally correct the author's apparent self-contradictions.
I don't want to sound excessively negative about this book. It contains a lot of interesting material and if one dips into it and reads here and there it can be rewarding. But read consecutively with the aim of extracting a coherent argument, it is hard work. It certainly would not be recommended for anyone who knew little about the subject and who wanted to get a grasp of what the Enlightenment was.