Each chapter costs a a key, takes 3 hours to get one key and has a maximum of two keys at a time. To top it off, if you want to do the chapter again, then it requires another key. And when you finish the book you can't go back and do it, without a bucket load of keys. Then there is the diamond, that are needed for outfits (otherwise outfits that make no sense). AND you need diamonds to unlock part of stories and/or to progress. To get diamonds is so minimal especially considering the need.