Secret origins / by James Riley.

Enhanced descriptions from Syndetics:

Bethany travels to a new fictional world to rescue her father in this third book in the New York Times bestselling series, Story Thieves -- which was called a "fast-paced, action-packed tale" by School Library Journal-- from the author of the Half Upon a Time trilogy.

Owen and Bethany have sworn off jumping into books for good. But they didn't make any promises about not jumping through strange portals that lead to a comic book world.

Jupiter City was once filled with brightly costumed superheroes and villains, but nowadays, there's nothing left but the Dark. Even the villains are terrified of the Dark's shadows, and most of the heroes have either disappeared or been lost to mind control. The one hero who might have stopped all of this, Doc Twilight, has been imprisoned by the Dark.

But who is Doc Twilight really? And how can Bethany and Owen defeat the Dark without superpowers of their own? They'll definitely need the help of some old friends and new allies to bring the light back to Jupiter City, and find out the truth behind the Dark.

It all comes back to Bethany's own secret origins. What really happened when her fictional father disappeared years ago? Who is Nobody, and why is he writing these Story Thieves books? And what kind of supervillain name is The Rotten Banana?