There are many synonyms that can be used instead of "following book." To start, "sequel" is an obvious choice as it directly implies a continuation of a story. "Subsequent volume" is a more formal option for a series of books, while "next installment" can be used for a serialized story. "Successor" can also be used in a broader sense to indicate a book that comes after another in a specific genre or author. Another option is "continuation," which can be used to indicate a book that picks up where the previous one left off. Overall, there are many great alternatives to "following book" that can add variety and nuance to your writing.