Answer:
(1) When John Boyne says that he loves getting lost in someone else's world when he reads novels, it means that he enjoys immersing himself in the story, characters, and settings of the book. He likes to escape from his own reality and enter into the imaginative world created by the author, where he can experience different perspectives and emotions.
(2) Another way to express this idea could be: John Boyne finds pleasure in losing himself in the fictional universe of a book, where he can mentally transport himself to a different place and time, and fully engage with the story and characters as if they were real. By doing so, he can temporarily forget about his own world and immerse himself in a new reality created by the author.