Raistlin’s spellbook

He leaned on his staff with one hand, holding open a book with the other. Tanis at first wondered what the mage was studying, then realized it was his spellbook. It is the curse of the magi that they must constantly study and recommit their spells to memory every day. The words of magic flame in the mind, then flicker and die when the spell is cast. Each spell burns up some of the magician’s physical and mental energy until he is totally exhausted and must rest before he can use his magic again.

Excerpt From Dragons of Autumn Twilight, pg 132
Margaret Weis & Tracy Hickman