You can either work it into the story line or source it.
If you work it into the story line, your character might say something like "As Dresden says in Butcher's books '“Yeah, um. I’m a wizard. I meddle. It’s what we do.' No. Really. That is what we do. To do anything else goes against our nature."
or you source it with a footnote. You can do that with footnotes or end notes. Or make a credit page with a list of the included quotes and the sources of them.