There is this book I really liked when I was 15, still think it's really good. It took me a few years to realize that the book is just a very elaborate rendition of a game of Werewolf (similar to Mafia).
I very much enjoy playing solo ttrpgs. I really don't subscribe myself to the "dude bro" energy of just writing a book because playing solo is something I don't understand. If I wanted to try to write a book then I would write a book. But that's the thing, I don't want to write a book. I want an experience that I can be both the GM and player with a story that can surprise me. I hardly even journal my sessions. Just general notes about monsters that I would fight or what I bought from an NPC. Just my 2 cents I guess.
For solo it is best to have open world instead of normal campaign. Because in an open world where there were no specifically decided goal or end, even the GM is playing/experiencing as much excitement/surprise as the player. Roll for encounter, roll for plot, roll for world development, roll for political move, etc. Just roll for everything and decide later how everything connects together.
Writing a book is when you want to present your gameplay in a more coherent manner. Which a standard campaign fortunately is.