Sure, Billy Bathgate is fiction. It falls within the realm of creative writing, where the author weaves a tale that may or may not have any resemblance to reality.
Jane is likely a fan - created character. She could be an ordinary girl who somehow gets transported into the Naruto universe and starts writing a diary about her new life there.
Billy Elliot is based on a true story to a certain extent. It incorporates real elements but may have fictionalized parts to enhance the drama and entertainment value.