Symbolism, Imagery, Allegory
Like Wisty, Whit is allowed to take one personal item to jail. In his case, his father gives him an old, blank journal. It doesn't seem all too exciting until it suddenly turns itself into something like an iPad:
"The book shows each of us what we want to see," Wisty observed. "It's magic. That's why Dad gave it to [Whit]." (31.18)
The book goes from being filled with blank pages to being filled with whatever pleases the person looking at it in a particular moment—so when it shows pretty ladies when Whit looks at it, we're reminded he's a hot-blooded teenage boy, no matter how much magic swirls around him.
Later, when Whit is reunited with his parents, the journal assumes its truest form. As Whit gazes into his father's eyes, "the journal filled in with lessons, explanations, magic spells—everything a witch and wizard would need to know" (103.23), transferring knowledge from father to son. This is similar to how Wisty's drumstick turns into a wand after she gazes into her mother's eyes.
And just as Wisty's mom tells her music is in her future, Whit's father tells him, "You're a very, very good wizard, Whit. And, believe it or not, you're going to be an important writer" (103.12). Just as Wisty is surprised by the musician revelation, Whit is shocked by his dad's declaration. "I thought the wizard thing was pretty out-there, Dad, but… a writer? You've got to be kidding" (103.13), he says. As readers, we are as surprised as Whit is since there's been no other mention of his writing until now.
Guess we'll have to read the other books in the series to see if his father's prediction proves correct.