The Riddle of the Wren — Charles de Lint

Inside the world of dreams

Minda Sealy is afraid to go to sleep, for when she does she is trapped inside the same terrible, dark nightmare. Over and over it happens, until she is ragged and raw. Then, one night, she dreams she is somewhere else -- walking a high, peaceful moor. There she meets Jan, the "heart of the moors," who has been imprisoned by Ildran the Dreammaster--the same being who controls Minda's nightmares.

In exchange for her promise to free him, Jan gives Minda three things: a new name, Talenyn, meaning "Little Wren"; a pouch of pebbles that can function as a gate between worlds; and an acorn pendant that will keep the evil Ildran from invading her dreams.

Minda sets out the very next night, leaving the safety of her old life to begin a journey from world to world, a journey both to save Jan and to solve "the riddle of the Wren"--which is the riddle of her very self.