Thank you for your question!
You write a tabletop role-playing game. Which can be any set of documents or even cards, not really a book. Most people just submit one or two PDF files. To get a better idea, you can check out all the entries of previous years, like:
https://itch.io/jam/rpgenesis2019/entries
https://itch.io/jam/rpgenesis2020/entries
https://itch.io/jam/rpgenesis2021/entries