We think about subjects for the authors we're already publishing. For example, Bonnie Shemie has done terrific books on native dwellings across North America. We think about special events and trends. For example, the Olympics are coming up again. Mostly, good ideas come from authors themselves.
Professional writers develop many techniques for ideas.
I've never met a good author who doesn't love to read. There's nothing like reading to make you think, and having good ideas is all about thinking. Writers often keep a journal, notebook, or files in which they record everything from character studies to a plot twist, to a description of a person or a term of phrase. I'm sure to remember it. Writers spend hours looking through reference books, old newspapers, magazines, letting themselves graze until ideas strike them.