Bishop Gottskálk the Cruel was the greatest wizard of his day; he gathered together all the black spells, which had never been used since the heathen times, and wrote them all down in a magic book called Red Skin. It was written in gold letters, and looked most magnificent in every way; it was written in runes, like other books of spells. The Bishop grudged that this book should pass to anyone else after his time, and therefore he had it buried with him, and he taught nobody all his lore.
For more information on Red Skin and how a crafty magician tried to retrieve it, see Jacqueline Simpson’s Legends of Icelandic Magicians.
(And no, I never did find out any more information about that spirit-summoning method described in the Icelandic sagas.)