"What is in the books" can be expressed in a variety of ways with different synonyms. It could be replaced with "the contents of the books," "the information within the books," or "the material contained in the books." Similarly, you could use "the knowledge presented in the books," "the subjects covered in the books," or "the topics explored in the books." Furthermore, you could say "the data recorded in the books," "the facts outlined in the books," or "the details cataloged in the books." All of these phrases convey the same meaning as "what is in the books" and could be used interchangeably depending on the context.