Let me start by saying I have not read this book cover to cover, but it's also not that kind of book. It's organized into chapters about specific aspects of HDM, allowing you to read the chapters relevant to your own interests or studies.
I have skimmed a few chapters, and overall the language and ideas are overly simplistic and geared toward young readers in a rather patronizing way.
I read the chapter on witches the whole way through, as it is the most pertinent to my thesis and this is the only source I've found so far that devotes a whole chapter to Pullman's witches.
Gresh's handling of actual witches is preposterous. A quick glance at the three-source biography for the chapter (two of which I'm pretty sure are essays, and one of which is an encyclopedia with no specific articles mentioned or anything) helps explain this, but it's still thoroughly disturbing. She mixes myths in with the truth, often in a way that begins with "Christians/witch hunters believed..." but by the end of the passage has taken on a tone of factuality. She blatantly insults modern witches, calling witchcraft a pseudo-religion and saying she has no idea why anyone would want this label but she suspects it makes girls feel cool to pretend they have supernatural powers. There are so many things wrong with that sentence I don't know where to begin.
She also grasps at straws a lot, using Gaia as a primary connection between Pullman's witches and actual witchcraft, even though Gaia is not explicitly part of either so much as a concept that both HDM and her brief reading about the history of witches seemed to remind her of. Later, she assumes Pullman's witches cast magical circles simply because she has found other similarities with "real" witches (quotation marks hers, not mine). Most of the points and conclusions she makes are completely unfounded, or else based in misinformation, stereotypes or outrageous myths (she actually says modern witches become invisible, shape shift, use toads and hares, etc.) Thus, even if you come well-equipped with a background of accurate information on the subject, it's basically impossible to take anything new away from her writing because less than half of it is based on conclusive evidence or arguable links between Pullman's and actual witches.
You would be much better off reading something like Margot Adler's Drawing Down the Moon and making your own connections to the witches in HDM; it would take less effort than trying to find anything of value on the subject here.