Jennet is an intelligent child somewhat improbably educated in natural philosophy by her aunt, who is soon enough burnt as a witch by Jennet's witchfinder father. Jennet undertakes to use the arts of natural philosophy and reason to prove the impossibility of witchcraft (or at least, maleficia) and thus effect the repeal of the Witchcraft Act. This pursuit consumes her life, which is itself quite eventful in a 1700 sort of way... transported to America, kidnapped by marauding aborigines, becoming part of native society, returning to tutor young Ben Franklin in the amatory arts, travelling to England to beseech Isaac Newton as an ally against demons, shipwrecked, 'rescued' by pirates, and hopefully the last witch trial on American soil, and so on. Meanwhile, her philosophical investigations ebb and flow.
It's quite a good historical novel; it hews close(*) to historical truth, and also close to the literary feel of the period. Her adventures could be something from Defoe. And of course Morrow's sympathies are with Reason over Superstition, though Reason has a tough row to hoe, as it did in actual history. Since I have creationism on the brain recently, it's hard not to consider parallels between the elimination of belief in witchcraft (or at least support for witch persecution) and what one sees from the creationist world. Morrow quotes the founder of Methodism: "Giving up witchcraft is, in effect, giving up the Bible." Much as creationists today require a literal Genesis and a literal Adam, lest they 'give up the Bible'. It might be useful as an analogy, but I worry about what I might find if I scratch a creationist about his or her views on witchcraft...
Morrow is better known for speculative fiction than historical fiction, and there's one element that is more on the speculative side. The book is occasionally narrated by the Principia Mathematica. We learn a bit about the secret life of books, and the war (mediated primarily by armies of silverfish, bookworms, and bibliophagous beetles) between the PM and the Malleus Maleficarum. I actually found the discursions on the book on book war a bit tedious, but the PM is actually a very perspicacious narrator of Jennet's story. These narrations are relatively brief and rare, ushered in by an interesting typographical effect.
The only drawback, if it be one, is that the book is long and leisurely. If I wanted to hook someone on Morrow, this is not the book I'd recommend. On the other hand, it may be his most subversively accessible defense of reason over superstition for the kind of people who most need it. No creationist is going to get past the title of Towing Jehovah or Only Begotten Daughter (though they should). But they might pick up this, and it might deftly get them to root for the correct side.