Not for clients, but for yourselves? Anything from short tiny ebooks to huge novels and even series?
Not only does writing books like this on a topic that you are good at niche-wise increase publicity for you, but it is also a really good supplemental income source. Passive Income, as it is called, as very minimal effort is required to make money after the book is written and published (aside from marketing, of course, but that’s a given)
I personally write some stories under a pseudonym, but I am potentially going to write a young adult book under my own name. However, do you writers write books and publish/self-publish?