This was a good overview of the subject, building on the many articles Hanegraaf has put out over the years. (He is very prolific.)
The book is unapologetically polemical: Hanegraaff wants to introduce and defend a relatively new field of study, that of Western esotericism, which he admits is kind of a grab bag of topics, from Renaissance Hermeticism to NeaoPlatonism to Crowleyian magic to (parts of) science fiction.
Part of Hanegraaff's point is to explain exactly why the field is a grab bag. In his telling, there's a grand narrative to Western religion: early Christianity defined itself against Paganism; the Reformation defined itself against Paganism, and the infusions of Paganism in earlier Christian cultures; and the Enlightenment defined itself against the "superstitious" elements it found within Christianity (not necessarily against Christianity itself). These things that comprised the other in each confrontation, these became varieties of esotericism. (Though Hanegraaff admits that term is limited, and not really descriptive of the many practices, it is the term scholars have agreed upon.)
It isn't that people stopped believing in and practicing these various forms of esotericism--they just became neglected as the academic disciplines congealed in the 18th and 19th centuries. Looking at these subjects was considered bad form--it was all superstition, the worst impulses of human culture. It was neglected, or forgotten knowledge. (As it happened, official dismissals of these various ideas helped to make them even more tempting to some people--no fruit tastes as good as the forbidden one.)
Eventually, by the mid-20th century some scholars did start to turn their attention to these subjects, particularly Frances A. Yates. (Hanegraaff doesn't spend much time on her, dismissing her work as riddled with errors.) As he sees it, though, the motivating factor of these early researches was ahistorical: what he calls religionist. The authors argued that there was a transcendent religious impulse in humanity that explained the pull toward esotericism. This explanation especially made sense in the 1960s and 1970s, when scholars were puzzling over why the secularization thesis had not worked out: why religion had not died away and, indeed was seeing a revival, both in terms of fundamentalism but also what was called the "occult revival." (Hanegraaff is surprisingly positive about the work of James Webb.)
We are now entering a third era in the study of esotericism, the one that Hanegraaff defends and defines. That is seeing esotericism not as a transcendent impulse, but as a historical category, a set of ideas and practices that change over time in relationship to the wider culture. Thus esotericism renovates in response to the Protestant Reformation, the Enlightenment, the development of professional science in the 19th century, and the triumph of instrumental reason in the 20th. The book, in part, charts these changes.
It is a short book, so many of these changes are just glossed. There are also some confusions of terminology--with Hanegraaff applying the categories "Theosophy" and "naturphilosophie" to much older traditions than I am used to seeing. He is very learned, and ranges across a wide array of cultural changes over a long term, and some of that ranging makes me nervous: it is often difficult to accept that dynamics laid down in the sixteenth century are still playing themselves out three hundred years later, for example. Nonetheless, it is all interesting and worth pondering.
Hanegraaff has a very categorical mind: it's what makes him easy to read, and his papers easy to follow. And this is shown in the second part of the book, chapters four through seven. The chapters themselves take up various themes, and these are broken down by category. Thus, in chapter four, he looks at world views inherent to esotericism--there are the two types of radicalism, monistic and dualistic, the second of which is more common. Then there are the ideas about what mediates between the dual spheres of the material and spiritual realms, NeoPlatonic sense that humans exist on a Great Chain of Being, between the animalistic and the divine; and the alchemical sense that transformation can bring the human closer to the divine. These various world views, of course, change over time.
He then turns to the various ways knowledge is created--which is a more recent innovation in scholarship, coming in these last thirty years, allowing that esotericists have defined methods of knowledge-making, even if they are not widely accepted. The methods are reason--not something exclusive to science--faith--which is also part of science, since many scientific principles are accepted on faith--and gnosis, which involves access to realms beyond the ability of humans to communicate and often rely on alterations in consciousness to reach.
Practices he divides into eight categories: (1) Control; (2) Knowledge; (3) Amplification; (4) Healing; (5) Progress; (6) Contact; (7) Unity; (8) Pleasure.
The penultimate chapter represents a third section of the book, taking these various structural elements and showing how they change over time in a process he calls "modernization"--but which dates to the Enlightenment. (The Enlightenment, for Hanegraaff, is a major dividing line.) He again divides the chapter into various categories. The first is correspondences, and he argues here, as he has in some of his papers, that while the theory of correspondences was held together by a belief in a central divine figure in pre-Enlightenment times, it is more arbitrary in later forms of esotericism. (I would have liked to see him engage with Alex Owen's work here, but it is never referenced.) A second transformation deals with the expanding horizons of religion, which in the past he has somewhat associated with secularism: by his lights, secularization means that Christianity becomes one of many different religious options, and in terms of this category he looks at how esotericists, especially in the 19th century, turned to other religions, especially those in the East. (He does not mention that many of these so-called religions, such as Buddhism, were not understood as religions within their own cultures.)
Evolution also becomes important; the 18th century bequeathed to us moderns the ideas that there could be spiritual progress and development. Evolution, as an idea, of course gets worked into esotericism in many ways---think of the Theosophists with their seven root races, for example. The development of psychology also plays a huge role in the history of Western esoteric traditions, as the interior or the mind becomes a place to explore, and the self something to realize. He ends this section with the idea of the "religious supermarket": that in the current environment, we are all free to choose are own religion, and shape it in ways we want. He offers some defintiional guides to understanding these practices, and makes some references to the most current forms of esotericism which completely negate the division between fact and fiction.
A final section is a useful, but incomplete, review of literature on the various topics he has covered. The annotated bibliography is heavy on European scholars of earlier periods, and light on research of more recent forms of esotericism. (Which may reflect Hanegraaff's idea, implied in the book, that the fragmentation of esotericism--the supermarket of faiths--means the the study of the phenomena needs to be taken up by various and separate disciplines.)
All in all, a good guide.