Here are just two instances of leaving money on the table.
You’ve just written some Web content for a client. In the process, you learned more about topic X than you ever wanted to know. Why not write up what you’ve learned into a nonfiction ebook?
When I asked Matt (a student) about his current writing, he said he was blogging for a client who sells health products. I knew he’d researched the products, the marketplace, and the audience, and asked whether he could turn his research into an ebook. We brainstormed angles, and within a few minutes, he had ideas for a couple of ebooks which he could not only sell, but which he could write in a few hours.