This is the sort of book that I’d really really like to get my hands on. Unfortunately, it’s a bit expensive (40 bucks even in paperback), so I’ll have to look around to find a copy in a university library around here.
It seems to figure that a lot of favorite medieval amulet-texts including Jesus, and a lot of healing sayings that essentially sound like pseudoepigrapha, are modelled on the devotion to the Letter of Abgar from the pilgrimage town of Edessa. There also seems to be quite a bit of interaction with private revelations, etc.
The sad thing is that a lot of these things would be common prayers and devotions one century, and would be denounced by the next. Of course, sometimes devotions really do go bad or overpopularity makes people go gaga for them. But there also seem to be a lot of cases where people come in and assume that simple people doing what they were taught must be up to something occult or heretical.
(And to be fair, some people who are big on devotions have real psychological problems, which tended to be something you could easily observe when people were allowed to practice devotions freely in churches. Nowadays these folks tend to have been chased onto the Internet, which probably allows me to underestimate how spooky priests can find the crazy people to be. But pastorally, I don’t really see how driving away all devotions out of fear of a few crazy people is any kind of solution. It’s probably better that crazy people should have something they can safely do in church with other people, and thus get some contact with other people as equals before God.)
There’s also a good amount of openly occult practices chronicled in the book. They pretty much were as depressing and stupid in the Middle Ages as they are today. And as David Drake showed in his Isles fantasy series, the voces magicae and the characteres still are pretty creepy as well as stupid.
There’s also the ars notoria, an example of bone laziness combined with wishful thinking. Real medieval students of the ars memoria drew pictures and diagrams (mentally or actually) as a focus for memorization and mental organization, while constantly going over their studies. Lazy medieval students of the ars notoria tried staring at pictures and diagrams (notae) which promised to magically teach you the liberal arts while not having to think about your studies, with the help of some prayers to angels and God at designated times. This was just as efficacious as sleeping on your schoolbooks (without tiring yourself out with study first), but I suppose it sold well to the gullible.
Another factor was interest in Hebrew as the ur-language taught by God, and hence intrinsically magical — which apparently led to Christian people buying Hebrew books and then cutting bits out for use as amulets. Greek was also exotic and inspired, even a little better than the sacred liturgical language of the learned, Latin. The Arabic language was seen as similarly exotic but not similarly powerful.