One of the books locked down here details how to break the boundary between the natural, arcane, and divine magics, and how to comprehend a spell you would not normally be able to learn.
Anyone capable of using a spellbook can use this tome to add any spell to their spell list. The spell must be of a level for which you have spell slots, and the tome can be used in this way only once. It then regains the ability to do so after one year has passed.