The phrase "book counterpart" refers to a book that is similar to a movie, TV show, or another type of media. It means that the book version of something matches or corresponds to its non-book version. Full definition
Then you have the superheroes themselves who not only mimic their comic book counterparts well but they almost look real in some strange surreal way. (impulsegamer.com)
The character was much different than his comic book counterpart in some aspects, but also exactly what we wanted him to be. (cbr.com)
We were told that there would be some differences between the movie version and his comic book counterpart, but that the essence of the character would still be intact. (cbr.com)