You don't need to be a company to publish books.

Freedom of speech is a right and publishing a book is an exercising of that right.

Actually you can publish books as a private person or as a Verein or as an Einfache Gesellschaft.

Anybody can register an ISB Number.

But if you want to be minimalist you probably don't even need that from a legal point of view. It just makes marketing easier, that's all.

So the short answer to your question is, there are no barriers, just go ahead and publish.

The down side of publishing without any legal structure is that you would have to take all income and stuff onto your private tax declaration, which means it could get a lot more complicated.

This is why many people form an Einfach Gesellschaft. All you need is a constitution, a bank account in the name of the Gesellschaft and a minimum of three officers, one of which can be yourself. I think as long as your turnover as an Einfache Gesellschaft remains below 20K per year you don't have to do taxes. But I would check this rather than taking my word for it.