I finally found a copy of Kieckhefer's "Forbidden Rites" and started reading it. Basically, it's a commented edition of a 15th century nigromancy (demonic magic) manual from central Europe (Munich).
It makes me wonder wether those who wrote these conjuration books seriously believed in their own tales, or if they simply wrote in order to please their audience and gain a better social standing (strange as it may seem in times when you could end in jail or worse for acquaintances with sorcery or heresy).
Many conjurations have futile purposes, such as organizing a fictive banquet where food does not relieve hunger, or creating a fake castle filled with fake (demonic) knights in order to simulate a fantasy siege. The common feature of these tricks is that they are aimed at a courtly audience. It's as if the purpose of the Munich manual was to seduce some powerful lord in order to become his personal magus.
There are strong folkloric overtones within the various conjuring tricks. For instance, when one conjures demons to set a fake banquet, he starts by classical drawing a magic circle on the ground. When the demons appear, clad as noble knights, they almost immediately invite the conjurer to taste their food, which he should take care not to accept. This pattern reminds me of various pieces of celtic folklore where eating food from the otherworld condemns one to never come back to the normal world. Then, the demons are said to start feasting, dancing and playing music. And again, the Munich manual warns the conjurer to refuse their invitation to join their dances. This also looks like a piece of traditional folklore : never leave a magic circle during a conjuration, never join the fairies in their dances ...
So, was the author of the book purposedly using folkloric tropes in order to please his audience, or did these folkloric stereotypes, still prevalent today, come from "nigromantic" sources ? I don't know, but I suspect the first option to be closer to truth. It would fit with the general purpose of gaining "social standing" at the court of some gullible noble.
In any case, I can't figure out how a rational person could find all this hassle and theoretical dangers worthwile. To me, it looks like a medieval hoax (or scam).
In one of the conjurations, the author explicitely says that at one point, you have to leave your clothes to the invoked demons, who will take them, and come to collect them back exactly one day later. If you fail to do so, or if you come too late, you'll die within a week. Where did he get this information ? Did he fail once to take back his clothes ? If that is so, how come he lived long enough to write his book ? It's obviously a tall tale from beginning to end. Not that I expected to find any profound esoteric knowledge in such a book, but to me this illustrates that the original author of the conjurations probably did not believe his own writings (or if he did, he was highly deluded).
The book is well worth a read though as an in depth case study in late medieval magic. Kieckhefer's commentary does make clear the expected profile of these medieval conjurers as coming from the clerical world. Their conjurations were imagined as distorted clerical rituals, and were quite similar to exorcisms. So the boundary between "straight" rituals and nigromancy was originally rather thin, and it was probably tempting for small underpriviledge clercs to profit from their ritual skills in an unusual way. Interesting.