I guess it goes back to the talk of shared illusions, shared worlds and stuff. You could say that all of the magical lore of Umineko could very well be Yasu’s ‘magical compendium’. Or maybe it’s more like, the existence of Mariage Sorciere as shared by Beatrice and Maria who both choose to believe in the legend of Beatrice.
EDIT: I can put it another way. Magic is a golden truth. Those who share the belief in it sustain it. Thus, a magical compendium is a system organised to sustain that belief. You could interpret things like the Stakes, the legend of the witch and the pentagrams as part of Yasu’s magical compendium. Those that share in that belief give magic form. It becomes truth, a golden truth.