Avalon: Cry of the Wolf (book 3) by Rachel Roberts, Softcover, Scholastic Books 2001.
Avalon Series Book 3. When Adriane's wolf, Storm, is lured back into the magical world, Adriane secretly follows. She is shocked by the things she sees, some wondrous, some terrifying. Her journey leads her one step closer to the source of all magic--Avalon--and to a boy she won't ever forget.