The Wizard's Dilemma: The Fifth Book in the Young Wizards Series

From Barnes & NobleThe Barnes & Noble Review
It's the wonderful world of wizardry, and adolescent spell casters Kit and Nita are about to face their biggest challenges yet in The Wizard's Dilemma, the fifth book in Diane Duane's magical Young Wizards series. Things between Kit and Nita are unusually tense these days, and they find themselves going their separate ways. Neither of them is happy about it, but they plug along as best they can, each of them exploring new realms of wizardly wonder. Then Nita's mother is diagnosed with cancer, throwing Nita's life into a total tailspin. For it seems that neither modern medicine nor ancient magic can cure this deadly disease...unless Nita makes a deal with the devil.
Kit and Nita both bide their time apart by exploring intriguing new alternate universes where things aren't always what they seem and the laws of physics and nature are often askew.