I have read the getting indexed page and am still wondering, what all content can I upload for sale here? 

I know games, music, and assets, but I was wondering, as a writer, can I upload my novels for sale here? Or are books limited primarily to custom/homebrew TTRPGs?

You can sell a novel on Itch.io, no problem. Not sure if there's much of a market, but you can upload a book about anything you want.