Hello, Cody and Chette!
I think the book there is just that, "a book" -- "written within and on the backside, sealed with seven seals".
Since the seven seals were opened in the following chapters, I guess the things written within and on the backside of that book are all found in chapters 6 - 8. Each time a seal is broken, somethng happens and John records them. These chapters must be the contents of the seven-sealed book.