When it comes to discussing the contents of a book, there are various phrases that can be used instead of "what is in the book". For instance, one can say "the book's content", "the book's information", or "the book's material". Other alternatives include "the book's substance", "the book's essence", or "the book's subject matter". Additionally, one can use phrases such as "the book's theme", "the book's plot", or "the book's storyline". In summary, when discussing a book's contents, there are various synonyms that can be used to add variety and depth to the conversation.