Mystekos Summer (Lake of Two Worlds - Book 1)

Emily Moss's mother is dying from some mysterious disease that is causing her to age beyond her years. Emily wants to stay with her mom, but instead, she is sent off for a week at Camp Tender-Heart. One night, she sneaks out of her cabin and finds a magic necklace that leads her to the world of Mystekos. Once there, she is asked to help defeat an evil witch who is trying to take over the world. Eventually, though, she sees how what is happening in Mystekos will help her learn to deal with what is happening in her own world.