In the library in Twilight Temple.
...chosen to explore this relation of world to shadow, Azra was the first to realize that shadows were not a mere absence of light, but a reflection of possible worlds created by forces in conflict. A light strikes a rock, and the shadow is a record of their clash, past, present and future. Other conflicting forces produced less obvious shadows, fire and water, wind and rock, or nations at war. With skill and patience the shadows of all could be read, and patterns teased out, emphasized or eradicated. Manipulating a shadow could, through contagion, manipulate the object or force which cast it. Azra was eminent master of this nearly imperceptible magic.