Heart of Valor — L.J. Smith

WHEN AN EARTHQUAKE SHAKES California: Alys, Charles, Janie, and Claudia suspect it isn't a typical earthquake. A year and a half after their journey into the Wildworld, they believe that the Passage between their world, the Stillworld, and that parallel universe of legend and danger may no longer be stable.

With their parents overseas and the great sorceress Morgana Shee traveling north to find the epicenter of the quake, the siblings are left on their own. But peril lurks around every corner. A magical attack sends the Hodges-Bradley kids on a journey that will test all their skills, including Janie's sorcery and Claudia's ability to communicate with animals.

Morgana's archrival, Thia Pendriel, is waiting. With Heart of Valor, the Forgotten Gem she has stolen, Thia's power is almost limitless -- and she is ready to spring her trap.