Story thieves

Except for reading the Kiel Gnomenfoot magic adventure series, Owen's life is boring until he sees his classmate Bethany climb out of a book in the school library and he learns that Bethany is half-fictional and has been searching every book she can find for her missing father, a fictional character.

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...

After Nobody destroys the connection between fictional and nonfictional worlds, causing nonfictional people to lose their imaginations, Owen and Bethany must risk everything to save multiple realities.