I finished this book a month ago, but I still find myself thinking over much of the information I got from it, and to me, that's the sign of a good book. Its take on the relationship between the witch and the familiar is unique, in my experience. For instance, Wilby shows that familiars were by no means "fetches" for witches and did not act as slaves or pets. They were equals in the relationship and could even act to the detriment of the witch. For me, the concept of moral ambiguity that Wilby posits for the familiar shows the familiar's depth and independence, its realness. It also fits with the position of the witch or cunning folk in their community - they were depended upon for medical help, communicating with the dead, or help in finding things, but they were also feared for the trouble they could cause to someone if they were antagonized.
The treatment of the fairy folk is also unusual. These are not Victorian Tinkerbells flitting around the garden like a bunch of beneficent mosquitoes. These are a powerful people with their own agenda, recognizable to those who've read British folklore or even stories by Arthur Machen. Wilby makes a pretty convincing argument that the fairies served the same purpose for early modern witches that guiding spirits have done for shamans in traditional societies. Like those spirits, fairy familiars helped witches acquire practical knowledge, like where a stolen cloak might be or how to cure (or hex) someone, and they were often friends and companions as well. Witches generally first encountered a familiar while being under extreme stress - broke, family members sick, overworked, hungry, fearing the worst - and Wilby compares this to the sort of deliberate preparation to encounter a guiding spirit that shamans in traditional societies engage in - fasting, depriving themselves of sleep, and creating other physical extremes. This interesting parallel fits with assertions made by Carlo Ginzburg in his work on the Sabbat, Night Journies. Wilby also argues that the concept of traveling to a sabbat is basically the interrogators' interpretation of the witch accompanying a fairy to fairyland, where for instance they might learn how to use plants or feast and dance with the fairy folk.
I was surprised by some of the information given in simple asides, such as that people at this time kept toads for pets or the average number of cats per household in Britain at this time was five. Something that really stuck with me, though, was how often witches described their familiar as being like an animal but not of any recognizable type, such as "something like a rabbit" which appeared in a witch's bed at night and asked him to love it. Even the pictures of them were somewhat disturbing. Other familiars appeared as humans but often wore old fashioned clothing.
This isn't the most engaging book ever written - the writing is just serviceable, and the book feels like it began life as a dissertation. But it does present what in my experience is a unique perspective on familiars. And the information here also goes far to bust up preconceived crusty notions about the good, wise hedgewitch who wouldn't hurt a fly, the Burning Times, Disneyworld-style fairies, and so forth. It presents a world much more complicated than that. The footnotes and bibliography also provide a number of directions for further reading about cunning folk in other countries as well as some interesting books on fairy folk in Britain.